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LONDON:   BLACKIE  &   SON,  LIMITED,  50  OLD   BAILEY,  E.C. 
GLASGOW,    AND    DUBLIN. 


Wfctorfan  J6ra  Series 


Recent  Advances  in  Astronomy 


Recent  Advances 


m 


Astronomy 


By 

ALFRED  H.  FISON,  D.Sc. 


LONDON 
BLACKIE  &  SON,  LIMITED,  50  OLD  BAILEY,  E.G. 


GLASGOW  AND  DUBLIN 
1808 


c/ 


'L  9- 


Preface 


In  the  following  pages  I  have  endeavoured  to 
give  a  simple  account  of  some  of  the  more  interest- 
ing "  Recent  Advances  in  Astronomy".  To  har- 
monize with  the  general  scheme  of  the  series  of  which 
this  work  forms  a  volume,  it  was  at  first  suggested 
that  I  should  develop  recent  progress  in  Astronomy 
historically.  The  difficulties  in  the  way  of  treating 
any  branch  of  science  in  such  a  manner  are,  how- 
ever, very  considerable ;  especially  when,  as  in  the 
present  instance,  it  is  desired  to  present  the  subject 
in  such  a  manner  as  to  be  readily  followed  by  those 
who  have  but  slight  familiarity  with  its  techni- 
calities. I  am  only  acquainted  with  one  entirely 
satisfactory  "  History  of  Astronomy",  and  that  one 
scarcely  appeals  to  other  than  professional  astro- 
nomers. It  has  upon  the  whole  appeared  best  to 
effect  a  compromise  between  an  historical  and  a 
purely  descriptive  method;  and  I  have,  therefore, 
while  dealing  with  what  have  appeared  to  me  to 
be  a  few  among  the  more  interesting  problems  of 
modern  Astronomy  in  a  series  of  separate  essays, 
followed  in  each  the  historical  method  as  far  as 
possible.  It  has  been  found  practicable  to  adhere 
to  this  scheme  more  rigidly  in  the  latter  part  of  the 
work. 


VI 


Preface 


Every  writer  of  a  popular  work  on  Astronomy,  or 
any  other  branch  of  science,  must  become  largely 
indebted  to  those  who  have  devoted  their  labour 
to  the  compilation  of  works  of  reference;  and  I 
would  acknowledge  my  deep  obligation  to  the  ex- 
tensive accumulation  of  accurate  knowledge  con- 
tained in  Miss  Clerke's  two  works — A  History  of 
Astronomy  during  the  Nineteenth  Century ,  and  The 
System  of  the  Stars. 

A.  H.  FISON. 

September,  1898. 


Contents 


CHAPTER  I 

Page 

The  Life  of  a  Star / 

APPENDIX   TO   CHAPTER  I 
The  Measurement  of  Stellar  Distances          -        -        -50 

CHAPTER  II 
The  Milky  Way  and  the  Distribution  of  Stars     -        -59 

CHAPTER  III 
The  Recent  Study  of  Mars 101 

CHAPTER  IV 
The  Analysis  of  Sunlight 144 

CHAPTER    V 
The  Analysis  of  Starlight 193 

CHAPTER    VI 
The  Red  Flames  of  the  Sun 219 

INDEX 239 


Recent  Advances  in 
Astronomy. 


Chapter   I. 
The  Life  of  a  Star. 

"Great  is  the  mystery  of  Space,  greater  is  the  mystery  of 
Time.  Either  mystery  grows  upon  man,  as  man  himself 
grows;  and  either  seems  to  be  a  function  of  the  godlike  which 
is  in  man.  In  reality,  the  depths  and  the  heights  which  are 
in  man,  the  depths  by  which  he  searches,  the  heights  by  which 
he  aspires,  are  but  projected  and  made  objective  externally  in 
the  three  dimensions  of  space  which  are  outside  of  him." 

DE  QUINCEY. 

With  our  present  knowledge  of  the  sun -like 
nature  of  the  stars,  and  the  colossal  part  that  they 
play  in  the  scheme  of  the  physical  universe,  it  ap- 
pears strange  that,  in  spite  of  the  bold  spirit  of 
speculation  that  characterized  the  ancient  philo- 
sophy,— a  philosophy  that  recognized  the  possibility 
of  the  development  of  higher  forms  of  life  from 
lower;  that  saw  in  the  Sun,  Moon,  and  Earth  differ- 
ent forms  of  air  in  different  stages  of  condensation ; 
and  in  the  universe  itself  the  working  of  a  fortuitous 
concourse  of  atoms, — no  worthy  speculation  should 
have  been  recorded  as  to  the  nature  of  the  stars. 

(M520)  A 


2  Recent  Advances  in  Astronomy. 

Alike  to  the  philosophers  of  Ancient  Greece,  and  to 
the  early  astronomers  of  Greece  and  Alexandria 
whose  lives  were  spent  in  tracing  with  deepest 
thought  and  rarest  skill  the  movements  of  the 
heavenly  bodies,  it  was  sufficient  that  the  stars  were 
points  of  fire,  each  set  in  its  place  in  the  concave  of 
the  firmament,  and  eternally  borne  by  it  in  diurnal 
revolution  round  the  central  Earth. 

It  is  unnecessary  to  do  more  than  very  briefly  re- 
view the  steps,  initiated  in  the  bold  speculations  of 
Copernicus  towards  the  middle  of  the  sixteenth 
century,  by  which  our  present  knowledge  of  the 
sun-like  nature  of  the  stars  has  been  attained. 
Copernicus  had  shown  it  to  be  probable  that  the 
Earth  is  one  of  the  planets,  a  group  of  small  bodies 
revolving,  each  in  its  own  period  or  year,  round  the 
central  Sun;  and  had  recognized,  as  the  logical 
consequence  of  his  scheme,  that  to  remain,  as  they 
appeared  to  remain,  unaffected  in  their  apparent 
positions  upon  the  celestial  vault  during  the  sup- 
posed annual  sweep  of  the  Earth  in  its  orbit,  the 
stars  must  be  vastly  more  remote  than  the  Sun; 
but  to  him  the  material  vault  of  heaven  had  merely 
been  thrown  farther  back,  and  the  stars  were  still 
points  of  fire  studding  its  concave  surface.  The 
bolder  and  direct  deduction — that  to  appear  un- 
affected in  direction  through  all  seasons  of  the  year, 
in  spite  of  the  enormous  displacement  in  the  posi- 
tion of  the  observer  upon  the  moving  Earth,  the 
stars  must  be  so  remote,  that,  to  be  visible  at  all, 
the  majority  of  them  must  be  bodies  of  the  same 
order  of  light-giving  power  with  the  Sun  itself — was 


The  Life  of  a  Star. 


recognized,  though  with  hesitation,  by  John  Kepler, 
and  was  for  the  first  time  fully  accepted  by  Galileo. 

It  will  be  advisable  to  present  the  principle  under- 
lying this  deduction  in  a  more  definite  form,  since 
the  thorough  comprehension  of  what  has  already 
been  achieved  by  it,  and  what  may  reasonably  be 
expected  from  its  application  in  the  future,  is  of 
fundamental  importance  in  the  astronomy  of  the 
stars.  It  involves  directly  the  only  method  that  has 
so  far  been  successfully  •/ 

applied  to  the  measure- 
ment of  the  distance  of 
a  star. 

Let  the  curve  in  fig. 
i  be  regarded  as  repre- 
senting the  orbit  of  the 
Earth  round  the  Sun, 
an  oblique  view  of  the 
nearly  circular  orbit, and 
suppose  that,  the  Earth 
being  in  the  position  in- 
dicated by  the  point  p,  P 
a  star  is  observed,  and 
that  the  direction  in 
which  it  is  seen  is  recorded  with  all  possible  ac- 
curacy. Let  the  straight  line  PA  indicate  this 
direction.  The  star  must  lie  somewhere  in  the 
direction  PA,  but  there  is  nothing  in  the  observation 
to  indicate  its  distance  from  A,  the  point  of  observa- 
tion. Six  months  later,  however,  the  Earth  will 
have  reached  the  position  indicated  by  Q,  having 
traversed  in  that  time  one-half  of  its  complete  orbit. 


Fig.  i. — Illustrating  Stellar  Parallax. 


4  Recent  Advances  in  Astronomy. 

Let  the  direction  in  which  the  star  now  appears  be 
observed,  and  let  it  be  recorded  by  the  line  QB. 
The  star  lies  therefore  in  the  direction  QB,  but  there 
is  nothing  in  the  last  observation  to  indicate  its 
exact  position  in  this  direction.  Since,  however, 
the  pair  of  observations  have  located  the  star  in  the 
directions  PA  and  QB,  it  must  be  situated  at  s,  their 
point  of  intersection,  and  the  geometry  of  the  figure 
at  once  gives  the  proportion  between  the  distance 
of  the  star  and  the  dimensions  of  the  Earth's  orbit. 

The  principle  of  this,  the  only  method  by  which 
the  distance  of  a  star  has  so  far  been  determined, 
cannot  but  appear  extremely  simple,  but  a  difficulty 
in  the  interpretation  of  its  application  appeared, 
when  to  Copernicus,  as  to  his  followers  for  nearly 
three  hundred  years,  the  lines  of  sight  PA  and  QB 
appeared  to  be  parallel,  showing  no  tendency  what- 
ever to  meet.  Thus,  the  first  observation  having 
established  PA  as  the  direction  of  the  star,  the  second 
would  give  the  parallel  line  QC,  and  not  a  direction 
such  as  QB,  sensibly  inclined  to  the  first. 

Assuming  the  fact  of  the  Earth's  journey  round 
the  Sun,  the  only  possible  interpretation  of  the 
apparent  parallelism  of  the  lines  of  sight  was  that 
the  stars  are  so  remote  that,  although  the  direc- 
tions PA  and  QC  are  inclined  toward  each  other, 
each  being  directed  to  the  star,  the  inclination  is 
so  slight  that  it  was  incapable  of  detection.  That 
this  was  the  true  explanation  appeared  more  and 
more  certain  as  the  truth  of  the  Copernican  system 
became  more  firmly  established;  and  in  the  con- 
viction that  success  was  possible,  and  as  instruments 


The  Life  of  a  Star.  5 

were  devised  by  the  employment  of  which  it  became 
possible  to  determine  with  an  ever-increasing  degree 
of  accuracy  the  direction  of  a  star,  the  search  after 
the  inclination  of  the  lines  of  sight  towards  a  star 
from  opposite  extremities  of  the  Earth's  orbit,  or  the 
"  parallax  of  a  star",  became  increasingly  keen.1 

It  is  a  matter  of  history  that  after  close  upon  three 
centuries  of  arduous  toil — toil  occasionally  rewarded 
by  unexpected  discoveries  of  the  greatest  interest 
and  of  the  farthest-reaching  importance,  though 
resulting  in  failure  so  far  as  the  immediate  object 
of  the  search  was  concerned — success  was  at  last 
achieved.  In  1838,  Bessel  of  Konigsberg  demon- 
strated, as  the  result  of  the  critical  examination  of  a 
great  number  of  observations,  that  a  certain  small 
star  in  the  constellation  of  the  Swan  did  appear  to 
experience  a  displacement  in  its  position  upon  the 
heavens  during  the  progress  of  the  year,  the  inclina- 
tion of  the  lines  of  sight  toward  it  from  the  two  most 
favourably  situated  positions  of  the  Earth  in  its  orbit 
being  estimated  at  nearly  two-thirds  of  a  second  of 
arc.  The  star,  in  itself  an  insignificant  member  of 
the  orbs  of  heaven,  thus  destined  from  its  associa- 
tion with  Bessel's  discovery  to  acquire  an  honour- 
able place  in  the  history  of  astronomy,  is  known  as 
6 1  Cygni ;  and  its  distance  as  deduced  from  Bessel's 
measurements  was  600,000  times  that  of  the  Sun. 
Bessel's  estimate  has,  however,  been  reduced  by 

1  The  parallax  of  a  star  is  more  exactly  defined  in  astronomy  as  one-half 
of  the  greatest  observable  inclination  of  the  lines  of  sight,  or  as  the  inclina- 
tion toward  each  other  of  two  straight  lines  to  the  star,  one  directed  from 
the  Sun  and  the  other  from  the  Earth  at  a  time  when  the  direction  of  the 
Earth,  as  seen  from  the  Sun,  makes  a  right  angle  with  that  of  the  star. 


6  Recent  Advances  in  Astronomy. 

more  recent  measurements,  carried  out  with  finer 
instrumental  appliances,  and  with  the  advantages 
arising  from  accumulated  experience,  to  440,000 
times  the  distance  of  the  Sun. 

It  would  be  scarcely  advisable  to  digress  here  into 
a  critical  examination  of  the  difficulties  that  have 
been  experienced  in  the  search  after  stellar  parallax, 
and  the  methods  by  which  they  have  been  in  part 
overcome;  but  the  subject  is  of  such  high  impor- 
tance in  the  astronomy  of  the  stars  that  I  have  ven- 
tured to  give,  in  the  form  of  an  appendix  to  the 
present  chapter,  a  rather  more  detailed  account  of 
Bessel's  discovery,  as  well  as  of  the  leading  features 
of  more  recent  work. 

Within  a  few  months  of  the  date  of  Bessel's  dis- 
covery, Professor  Henderson,  of  Edinburgh,  an- 
nounced the  fact  of  his  having  succeeded  in  detect- 
ing parallax  in  the  bright  southern  star  a  Centauri. 
The  observations  in  which  the  parallax  of  the  star 
was  recorded  had  been  made  by  Henderson  six 
years  previously,  and  for  a  different  purpose,  during 
the  course  of  his  work  at  the  Cape  of  Good  Hope, 
and  his  attention  was  only  redirected  to  them  with 
a  view  to  the  investigation  of  parallax  by  the  an- 
nouncement of  Bessel's  success.  The  distance  of 
a  Centauri,  deduced  from  the  parallax  originally 
announced  by  Henderson,  is  180,000  times  that  of 
the  Sun,  but  the  most  recent  measurements  have 
extended  it  to  270,000  times  the  distance  of  the  Sun. 

So  far,  no  star  has  been  found  to  lie  nearer  to  the 
solar  system  than  a  Centauri.  A  vague  suggestion 
of  the  unthinkable  void  that  separates  us  from  this, 


The  Life  of  a  Star.  7 

in  all  probability  the  nearest  of  the  stars,  may 
perhaps  be  obtained  from  the  fact,  that  upon  such 
a  scale  that  the  orbit  of  the  earth  should  be  repre- 
sented by  the  circumference  of  a  shilling,  the  star 
would  be  removed  to  a  distance  of  two  miles. 
Across  such  a  distance  as  that  of  a  Centauri,  light, 
travelling  with  a  velocity  of  187,000  miles  in  a 
second  of  time,  would  speed  onward  for  four  and  a 
half  years ;  so  that  the  star  is  seen,  not  as  it  is  now, 
but  as  it  was  four  and  a  half  years  since,  while 
another  equal  period  must  pass  before  the  rays  now 
leaving  it  will  bring  their  record  to  the  shores  of  the 
Earth. 

The  discoveries  of  Bessel  and  Henderson  im- 
parted new  life  to  the  search  after  the  parallaxes  of 
stars.  So  delicate,  however,  are  the  necessary 
observations  of  direction,  and  so  many  and  serious 
are  the  sources  of  error,  that,  excepting  a  few 
isolated  successes,  the  record  of  the  next  forty 
years  is  chiefly  one  of  accumulation  of  experience ; 
and  when  in  1881  Dr.  Gill  and  Dr.  Elkin  com- 
menced a  series  of  observations  at  the  Cape  of 
Good  Hope,  the  parallaxes  of  not  more  than  half 
a  dozen  stars  had  been  detected  with  certainty. 
Since  that  date,  however,  parallax  hunters  have 
been  better  rewarded,  though  up  to  the  present 
time  it  is  doubtful  whether  success  has  been 
achieved  in  more  than  fifty  instances. 

Of  the  stars  the  parallaxes  of  which  have  been 
detected,  Sirius  is  undoubtedly  the  most  important. 
As  in  the  case  of  a  Centauri,  its  parallax  is  indicated 
with  some  degree  of  probability  in  observations 


8  Recent  Advances  in  Astronomy. 

made  by  Henderson  in  1832.  For  another  half- 
century,  however,  the  numerous  attacks  made  upon 
it  were  chiefly  remarkable  for  the  discordance  of 
their  results,  discordance  that  ultimately  vanished 
in  the  frequently  repeated  observations  that  have 
been  made  at  the  Cape  since  1891.  The  most 
recent  estimate — published  by  Gill  in  1898 — of  a 
parallax  of  '37  of  a  second  of  arc,  agrees  very 
closely  with  his  previous  results,  and  indicates  for 
the  star  a  distance  of  556,000  times  that  of  the  Sun. 

The  mere  statement  of  the  distances  of  stars  is 
apt  to  be  productive  of  weariness  of  the  spirit;  in 
their  absolute  magnitudes  they  are  entirely  un- 
thinkable, but  in  their  relation  to  things  familiar, 
they  may  well  produce  a  powerful  impression  of  the 
nothingness  of  the  Earth — so  far  as  its  physical 
relations  are  concerned — to  the  scheme  of  the 
physical  universe.  From  the  days  of  the  Psalmist 
it  has  been  customary  to  regard  the  heavens  as 
inspiring  a  sense  of  deep  humility  in  man,  but 
whether  Nature  in  her  most  sublime  aspect  would 
appeal  to  one  who  had  not  already  learnt  the  lesson 
from  communion  with  his  fellows  is  doubtful. 

The  chief  interest  of  the  distances  of  the  stars  in 
the  present  connection  lies  in  the  view  to  which  they 
necessarily  lead  regarding  the  nature  of  the  stars 
themselves.  If  it  were  removed  to  the  distance  of 
Sirius,  the  Sun  itself  would  fade  into  insignificance, 
shining  but  as  a  star  of  the  third  magnitude,  rather 
less  conspicuously  than  the  brighter  ones  that  form 
the  familiar  W  of  Cassiopeia.  Seventy-five  such 
stars  would  be  necessary  to  supply  light  equal  to 


The  Life  of  a  Star.  9 

that  received  from  Sirius ;  hence,  in  the  intensity  of 
its  light  radiations,  Sirius  exceeds  the  Sun  75  times. 
Of  the  few  stars  in  which  parallaxes  have  so  far 
been  detected,  to  appear  with  their  actual  luminosi- 
ties at  their  estimated  distances,  many  must  far 
exceed  the  Sun  in  light-giving  power,  while  a  few 
must  surpass  even  Sirius  itself.  Others,  however, 
and  among  them  Bessel's  star  in  the  Swan,  while 
undoubtedly  suns,  would  appear  but  as  modest 
specimens  of  their  class  if  placed  beside  ours,  and 
it  is  scarcely  possible  so  far  to  decide  whether  our 
Sun  reaches  the  average  of  splendour  displayed  by 
the  suns  of  space,  or  whether  he  surpasses  it.  It 
will  be  seen  in  a  later  chapter  that  the  sun-like 
nature  of  the  stars  is  further  indicated  in  the  analysis 
of  their  light  by  the  spectroscope. 

When,  as  is  the  case  in  the  overwhelming  ma- 
jority of  instances,  no  parallax  can  be  detected  in  a 
star,  its  distance  is  of  course  indeterminate,  but  it 
is  possible  to  assign  a  minimum  distance  beyond 
which  it  must  be  situated,  if  the  smallest  angle  of 
parallax  that  could  escape  detection  is  known.  So 
much  depends  upon  the  skill  of  the  observer,  upon 
the  position  of  the  star  in  relation  to  others,  and 
even  upon  its  colour,  that  it  is  not  possible  to  give 
any  definite  and  general  estimate  of  this  maximum 
parallax.  According  to  a  recent  statement  of  Dr. 
Gill,  however,  than  whom  undoubtedly  there  can 
be  no  higher  authority,  under  favourable  conditions 
a  parallax  of  a  fiftieth  of  a  second  of  arc  should  not 
escape  detection,  one  that  corresponds  to  a  distance 
of  rather  more  than  10,000,000  of  times  that  of  the 


io  Recent  Advances  in  Astronomy. 

Sun.  There  is  little  doubt  that  but  an  insignificant 
fraction  of  the  stellar  host  lie  within  this  limit. 

To  apostrophize  upon  the  picture  of  the  physical 
universe  revealed  by  these  discoveries  is  an  old 
story.  The  concave  vault  of  the  Old  Astronomy 
has  dissolved,  and  has  revealed,  beyond,  a  scheme 
unthinkable  in  its  vastness,  and  in  its  suns  and 
systems  of  suns  radiant  with  energy.  There  are 
few  among  us  who  have  not  experienced,  as  in  our 
more  emotional  moments  we  have  endeavoured  to 
penetrate,  however  superficially,  the  inward  mystery 
of  so  majestic  a  scheme,  and  one  in  which  man  plays 
apparently  so  humble  a  part,  a  sense  of  oppression. 
We  have  been  overwhelmed  with  the  sense  of  in- 
scrutable and  immanent  mystery;  and  we  have 
been  ready  to  exclaim  with  the  pilgrim  of  German 
fable,  "I  will  go  no  farther;  for  the  spirit  of  man 
acheth  with  this  infinity.  Insufferable  is  the  glory 
of  God !  Let  me  lie  down  in  the  grave  and  hide  me 
from  the  persecution  of  the  Infinite,  for  end,  I  see, 
there  is  none!" 

The  stars,  then,  are  suns ;  and  the  life  of  a  star  is 
the  life  of  a  sun.  Life  is  essentially  a  succession 
of  changes,  a  passage  through  varying  conditions 
of  activity ;  death  is  cessation  of  all  activity.  Are 
there  grounds  for  regarding  our  sun  as  undergoing 
change?  and  if  there  are,  what  is  the  nature  of  that 
change?  Are  there  indications  that  in  time  the 
activity  of  the  Sun  will  cease?  The  Sun  is  clearly 
a  hot  body  continually  throwing  off  an  enormous 
amount  of  heat  into  space  by  the  process  of  radia- 
tion. Unless,  therefore,  by  some  undiscovered  and 


The  Life  of  a  Star.  n 

entirely  unsuspected  process  an  equivalent  amount 
is  supplied  to  it  from  some  external  source,  it  must 
be  becoming  continually  poorer  in  its  store  of  heat.1 
In  the  remote  past  it  must  have  contained  far  more, 
and  in  the  distant  future  it  will  contain  far  less  heat 
than  it  contains  at  the  present  time.  Everything 
goes  to  support  the  straightforward  view  that  the 
light  of  the  Sun  is  the  direct  result  of  a  vivid  state 
of  incandescence  of  its  surface  consequent  upon  the 
high  temperature  to  which  it  is  raised.  As  the  Sun 
cools,  a  time  must  come  when,  unless  some  catas- 
trophe intervenes,  its  temperature  will  have  fallen 
so  much  that  its  beams  will  have  lost  their  present 
glory;  later,  it  will  cease  to  glow;  and  thereafter, 
as  a  dark  star,  a  sad  memorial  of  its  present  splen- 
dour, it  will  pursue  its  lifeless  course  through  the 
ages. 

Attempts  have  been  made  to  estimate  the  time 
that  must  elapse  before  the  period  of  this  Sun-death, 
but  in  our  ignorance  of  the  physical  constitution  of 
the  Sun,  and  more  especially  of  that  of  its  interior, 
such  estimates  are  affected  by  a  very  wide  margin 
of  uncertainty.  There  is,  however,  no  doubt  that 
the  actual  heat  existing  as  such  in  the  Sun  forms 
but  an  insignificant  fraction  of  its  total  store  of 
radiation.  Without  doubt,  the  Sun  is  largely,  if 
not  almost  wholly,  gaseous :  and  since  all  gases,  as 
also  nearly  all  solids  and  liquids,  expand  with 

i  A  rather  attractive  speculation  of  Julius  Mayer,  vigorously  supported  for 
a  time  by  Tyndall,  sought  to  account  for  the  maintenance  of  the  solar 
radiation  by  heat  developed  from  the  destruction  of  motion  of  meteorites  con- 
tinually falling  into  the  Sun.  It  has,  however,  been  shown  that  any  heat  that 
the  Sun  may  possibly  gain  in  this  way  must  be  quite  negligible  in  quantity. 


12  Recent  Advances  in  Astronomy. 

accession  of  heat  and  contract  upon  loss  of  it,  the 
sun  must  be  shrinking.  Further,  the  interior  of  the 
Sun  must  be  enormously  compressed  by  the  weight 
of  superincumbent  matter;  and  as  shrinkage  takes 
place  it  must  become  still  more  compressed.  But 
the  act  of  compression  of  a  gas  produces  heat; 
hence  heat  is  continually  being  generated  in  the 
Sun  by  the  compression  of  its  substance.  Each 
step  in  the  loss  of  heat,  therefore,  calls  into  exist- 
ence other  heat;  and  this  may  partly,  wholly,  or 
even  for  a  time  more  than  compensate,  for  the  loss. 
Subjecting  these  principles  to  mathematical  expres- 
sion, Helmholtz  has  shown  that  the  heat  thus 
evolved  by  compression  of  the  Sun  consequent 
upon  its  shrinkage  must  be  sufficient  in  amount 
to  maintain  it  as  a  self-luminous  body  for  many 
millions  of  years.  It  is  unnecessary  for  our  present 
purpose  to  attempt  to  arrive  at  a  more  definite 
estimate  of  the  future  of  the  life  of  the  Sun. 

Returning  to  the  present  condition  of  its  system, 
and  from  it  projecting  our  thought  backward  into 
the  past,  we  see  the  Sun  richer  and  richer  in  heat 
during  receding  ages;  becoming  more  perfectly 
gaseous — the  ultimate  effect  of  accession  of  heat 
being  to  convert  all  things  into  the  gaseous  state — 
while  ever  increasing  in  volume ;  the  planets  one  by 
one  disappear  in  its  expanding  bulk;  and  there 
appears  as  the  first  stage  of  Sun-life  a  diffused  body 
of  gas,  extending  beyond  the  present  limits  of  the 
planetary  system,  and  containing  latent  in  itself  a 
store  of  energy  that  is  through  coming  ages  to 
maintain  the  vitality  of  worlds. 


The  Life  of  a  Star.  13 

The  Sun,  and,  by  the  same  process  of  reasoning, 
the  stars,  would  thus  appear  to  have  originated  in 
extended  volumes  of  tenuous  gas,  and  to  be  fated 
in  the  end  to  be  degraded  into  cold  inert  masses. 
These  conclusions  being  accepted,  it  would  appear 
probable  that  both  of  these  conditions  would  be  at 
the  present  time  represented  among  celestial  bodies, 
for  even  upon  the  extreme  assumption  that  all  of 
them  were  created  at  the  same  time  and  in  the 
same  stage  of  development,  it  would  follow  that, 
since  they  differ  enormously  in  mass,  they  would 
cool  and  therefore  pass  through  their  life  stages 
at  different  rates.  It  becomes,  therefore,  of  great 
interest  to  inquire  whether  there  exist  in  celestial 
space  extensive  bodies  of  gas,  and  whether  there 
exist  dark  stars.  The  answer  is  clear :  astronomical 
observation  has  revealed  both. 

There  can  be  little  doubt  that  the  earliest  stage  of 
star-life  is  represented  in,  at  any  rate  many  of,  the 
nebulas.  The  nebulas  appear  as  faint  clouds  of 
light,  and  are  distributed  in  thousands  over  the 
face  of  the  heavens.  The  greater  number  are  ex- 
cessively faint,  their  very  detection  demanding  the 
aid  of  the  highest  optical  power;  while  two  only, 
and  those  just  hovering  upon  the  verge  of  vision, 
are  visible  to  the  eye  upon  the  darkest  and  clearest 
nights.  These  are  the  glorious  objects  in  the 
constellations  of  Andromeda  and  Orion,  the  one  in 
Orion  being  the  more  impressive  of  the  two. 

The  Great  Nebula  of  Orion  is  situated  near  the 
centre  of  a  line  of  faint  stars  that  trail  southward 
from  the  middle  of  a  line  formed  by  the  three 


14  Recent  Advances  in  Astronomy. 

bright  ones  that  constitute  the  "belt"  of  the 
familiar  winter  constellation  Orion.  Visible  to 
the  naked  eye  under  favourable  conditions  as  a 

faint  mist — 

A  single  misty  star, 
Which  is  the  second  in  a  line  of  stars 
That  form  a  sword  beneath  a  belt  of  three ; — 

its  cloudy  nature  clearly  revealed  in  a  hand  tele- 
scope or  a  good  field-glass ;  when  viewed  through 
a  telescope  of  large  light-grasping  power  it  becomes 
one  of  the  most  impressive  of  natural  objects, 
though  the  vast  extension  of  the  heavens  into 
which  its  wreaths  are  thrown,  and  the  abundance 
of  delicate  detail  permeating  the  whole,  have  only 
become  revealed  in  recent  records  of  the  photo- 
graphic plate. 

The  Nebula  of  Orion  appears  through  a  fine 
telescope  as  a  faint  green  haze,  suggesting  a  light 
cloud  floating  in  celestial  space,  in  form  not  very 
unlike  that  of  the  profile  of  a  fish's  mouth.  The 
whole  is  composed  of  clouds  of  light  of  different 
degrees  of  brightness,  some  of  extreme  fantastic, 
and  not  a  few  of  highly  suggestive  forms.  It  is  in 
the  perception  of  these  that  the  photographic  plate 
has  demonstrated,  as  powerfully  as  in  any  of  its 
applications,  its  great  superiority  over  the  eye  in  its 
capacity  of  appreciating  the  faintest  shades  of  light. 
Structure  is  revealed  throughout  the  whole  nebula 
by  the  manner  in  which  streams  of  luminous  matter 
are  directed  from  a  brilliant  and  nearly  central 
region  in  close  proximity  to  the  mouth-like  bay  of 
dark  sky,  the  importance  of  this  region  being 


The  Life  of  a  Star.  15 

emphasized  by  the  occurrence  in  it  of  a  remarkable 
group  of  stars — "  the  trapezium  of  Orion  " — and  in 
the  symmetrical  arrangement  of  many  of  the  cloud- 
forms  with  reference  to  it.  Many  stars  are  scattered 
over  the  picture;  that  those  of  the  trapezium  are 
actually  involved  in  the  glowing  wreaths  of  the 
nebula  itself,  and  do  not  owe  their  appearance  in  it 
to  the  effect  of  optical  projection,  either  by  their 
lying  by  chance  in  the  line  of  sight  towards  the 
nebula,  or  by  being  visible  through  its  transparent 
substance  while  actually  far  beyond,  is  rendered 
overwhelmingly  probable  from  their  position  with 
reference  to  the  cloud-forms,  as  well  as  by  certain 
relations  that  have  been  shown  to  exist  between 
their  analysed  light  and  that  of  the  immediately 
surrounding  nebula,  in  the  spectroscopic  researches 
of  Sir  William  Huggins. 

The  diffuse  character  of  the  outlines  of  the  nebula 
renders  it  impossible  to  apply  to  it  such  delicate 
measurements  of  direction  as  are  necessary  for  the 
determination  of  the  parallax.  For  this  reason  its 
distance  cannot  be  directly  investigated.  The  stars 
of  the  trapezium  have,  however,  shown  no  parallax; 
from  this  it  becomes  possible  to  assign  roughly  a 
minimum  limit  beyond  which  they,  and  therefore 
in  all  probability  the  nebula,  must  lie.  Such 
distance  can  scarcely  be  less  than  a  million  times 
that  of  the  Sun.  To  appear  of  its  vast  extent,  even 
at  this,  the  most  modest  estimate,  its  glowing 
clouds  must  extend  over  such  abysmal  depths,  that 
the  whole  of  the  Solar  System  if  plunged  into  it  would 
become  contemptible  in  its  utter  insignificance. 


16  Recent  Advances  in  Astronomy. 

The  Nebula  of  Orion  is  a  noble  example  of  an 
"  irregular  nebula".  That  of  Andromeda,  in  its 
regular  ellipticity  of  outline,  in  the  uniformity  in 
the  central  condensation  of  its  light,  and  in  the 
system  of  elliptical  rings  by  which  it  is  enveloped, 
forms  so  strong  a  contrast  with  it  that  it  is  difficult 
to  regard  the  two  as  objects  belonging  to  the  same 
class.  Other  nebulas  display  a  spiral  structure; 
others  again  appear  as  fairly  sharply  defined  plan- 
etary discs ;  while  the  majority  are  to  all  appearance 
nothing  more  than  minute  structureless  clouds  of 
flocculent  light. 

In  the  early  period  of  their  discovery,  a  discovery 
that  followed  naturally  upon  Galileo's  first  applica- 
tion of  the  telescope  to  astronomical  observation  in 
1609,  nebulas  were  regarded  as  diffusions  of  a  lucid 
medium  shining  by  its  own  inherent  lustre.  In 
1780,  the  year  that  marked  the  commencement  of 
Sir  William  Herschel's  classical  researches  upon 
them,  less  than  150  were  known;  but  as  the  result 
of  those  researches,  which  extended  over  a  period 
of  twenty-one  years,  their  number  had  been  in- 
creased to  close  upon  2500.  By  their  extended 
distribution  in  space,  as  well  as  by  the  detailed 
structure  revealed  in  many  of  them  by  Herschel's 
observations,  the  nebulas  had  acquired  a  new  im- 
portance in  the  system  of  the  Universe. 

From  a  not  altogether  satisfactory  deduction  from 
the  universality  of  gravitation,  an  extension  of 
natural  law  that  his  own  discovery  of  the  mutual 
revolution  of  the  components  of  double  stars  went 
far  to  establish,  Herschel  was  led,  in  the  earlier 


The  Life  of  a  Star.  17 

period  of  his  researches,  to  reject  the  generally 
received  view  regarding  the  nature  of  nebulae,  and 
to  substitute  for  it  one  according  to  which  they  were 
clusters  of  stars,  the  component  stars  being  too 
faint,  by  reason,  it  was  supposed,  of  excessive  dis- 
tance, for  their  individuality  to  be  recognized. 
While  maintaining  this  view  with  regard  to  the 
constitution  of  some  nebulas,  Herschel,  however, 
subsequently  reverted  to  the  former  hypothesis  to 
account  for  many  of  them,  these  including  the 
Nebula  of  Orion,  regarding  them  as  "  extensions 
of  a  shining  fluid  of  a  nature  unknown  to  us  ".  He 
further  framed  a  first  consistent  scheme  of  stellar 
evolution,  in  suggesting  that  individual  stars  and 
clusters  of  stars  were  formed  by  the  condensation 
of  this  nebula  substance  by  the  power  of  gravita- 
tion. 

During  the  first  half  of  the  present  century 
scientific  opinion  entirely  reverted  to  the  earlier 
of  Herschel's  views.  Changes  in  the  outlines  of 
certain  nebulas,  and  the  absence  of  structure  in 
others  of  the  " planetary"  class,  both  of  which 
Herschel,  thinking  that  he  had  established  by 
observation,  had  advanced  in  support  of  his  later 
views,  failed  to  receive  confirmation  in  their  ex- 
amination by  later  astronomers.  As  with  increased 
telescopic  power  many  objects  classed  as  nebulas 
were  one  by  one  resolved  into  collections  of  stars, 
the  conviction  became  increasingly  strong,  that, 
with  sufficiently  refined  means,  all  would  ultimately 
succumb:  and  when  at  length,  in  1850,  the  Great 
Nebula  of  Orion  was  thought,  from  its  appearance 

(M620)  B 


i8  Recent  Advances  in  Astronomy. 

in  the  gigantic  telescope  of  Lord  Rosse,  to  show 
indications  of  breaking  into  clouds  of  stars,  the 
riddle  of  the  nebulae  appeared  to  be  approaching  its 
last  solution.  As  clusters  of  stars  the  nebulae  found 
ready  place  in  the  speculations  of  many  astrono- 
mers, whose  minds,  in  consequence  of  the  perfection 
displayed  in  the  relations  between  the  Sun  and 
planets,  had  become  powerfully  impressed  with  the 
conception  of  a  system  as  an  essential  unit  in  the 
construction  of  the  universe.  The  planets,  with 
their  attendant  satellites,  formed  systems,  fair 
images  of  the  grander  Solar  System,  in  which  they 
were  included.  Each  star  was  regarded  as  a  sun, 
the  centre  of  a  planetary  system  of  its  own.  Visible 
isolated  stars  formed  with  our  Sun  a  larger  but 
essentially  similar  system  or  "galaxy",  in  which  it 
was  even  conjectured  that  all  members  might  re- 
volve round  a  central  orb ;  while  nebulae  were  other 
systems  of  suns,  external  galaxies,  awfully  remote 
from  our  galaxy  and  from  each  other;  oases  of 
active  energy  scattered  through  space.  The  de- 
molition of  this  stupendous  conception  by  later 
researches  has  been  advanced  as  supplying  the 
only  instance  in  which  astronomical  discovery  has 
failed  to  reveal  in  the  actual  a  more  majestic  scheme 
than  had  previously  been  idealized  in  the  boldest 
imagination. 

While,  however,  the  colossal  reflector  of  the 
Earl  of  Rosse  was  engaged,  it  was  fondly  believed, 
in  finally  establishing  the  nebulae  as  clusters  of 
faint  stars,  the  researches  of  Angstrom,  Bunsen, 
Kirchhoff,  and  others  were  placing  upon  a  firm 


The  Life  of  a  Star.  19 

foundation  the  principles  of  a  new  science  that  was 
shortly  to  enter  the  arena,  with  the  result  of  utterly 
confounding  general  expectation.  The  develop- 
ment of  the  science  of  Spectrum  Analysis  forms  the 
subject  of  a  later  chapter  of  the  present  work. 
Here  it  must  be  sufficient  to  record  that  as  early 
as  1672  Sir  Isaac  Newton  had  shown  that,  upon 
passing  a  ray  of  sunlight  through  a  glass  prism,  it 
became  separated  into  its  constituent  colours,  by 
reason  of  the  fact  that  all  rays  are  deflected  or 
u  refracted  "  on  traversing  the  prism,  but  that  rays 
of  different  colours  are  refracted  to  different  degrees; 
that  after  the  lapse  of  a  century  and  a  half  the  study 
of  the  analysis  of  light  was  resumed  and  the  instru- 
mental means  greatly  improved  by  Fraunhofer  of 
Munich ;  and  that,  by  the  labours  of  Kirchhoff  and 
Bunsen,  the  spectroscope  assumed  its  place  as  a 
powerful  instrument  of  research  about  the  year 
1860. 

The  spectroscope  is  essentially  an  instrument 
whereby  light  consisting  of  a  mixture  of  colours 
is,  after  entering  the  instrument  by  a  narrow  slit, 
resolved  into  its  constituent  colours  by  a  prism,  or 
occasionally  by  an  equivalent  " diffraction  grating". 
The  separated  colours  are  in  either  case  spread  out 
into  a  tinted  band  or  " spectrum".  About  the 
middle  of  the  present  century  observations  with  the 
spectroscope  had  indicated  that  there  was  a  remark- 
able difference  between  light  emitted  by  a  glowing 
gas  and  that  radiated  from  an  incandescent  solid 
or  liquid  body.  With  light  emanating  from  an 
incandescent  solid  or  liquid,  such  as  that  emitted 


20  Recent  Advances  in  Astronomy. 

by  a  glowing  mass  of  white-hot  metal,  or  by  a  gas 
flame  in  which  the  greater  part  of  the  luminosity 
is  due  to  incandescent  clouds  of  soot  deposited  in 
the  flame  from  the  decomposition  of  the  gas  under 
the  intense  heat  of  combustion,  and,  with  a  limita- 
tion to  be  noticed  subsequently,  that  from  the  Sun 
and  from  the  great  majority  of  the  stars,  the  spec- 
trum consists  of  a  continuous  band  in  which  all  the 
colours  of  the  rainbow  are  represented,  each  passing 
into  the  next  by  insensible  gradations,  while  red 
and  violet  occupy  the  extreme  positions.  In  the 
light  from  a  glowing  gas,  however,  at  any  rate 
when  the  density  of  the  gas  is  not  excessive,  this  is 
not  the  case,  the  light  being  now  resolved  into  a 
series  of  clearly-defined  and  separate  colours,  which 
appear  in  the  spectroscope  as  bright  lines  of 
coloured  light  separated  by  dark  intervals;  the 
lines  are,  in  fact,  images  of  the  slit  by  which  the 
light  enters  the  instrument,  a  separate  image  being 
formed  by  each  of  the  colours  present.  The  light 
from  the  flame  of  a  spirit-lamp  which  has  acquired 
a  strong  yellow  tint  by  sprinkling  a  trace  of  com- 
mon salt  upon  the  wick,  is,  for  instance,  resolved 
into  two  closely  coincident  shades  of  yellow,  indi- 
cated in  the  spectroscope  by  the  appearance  of  a 
pair  of  closely  adjacent  yellow  lines;  and  the  peach- 
coloured  glow  emitted  by  hydrogen  gas  when 
rendered  luminous  by  a  discharge  of  electricity 
through  it,  gives  rises  to  the  appearance  of  several 
coloured  lines,  of  which  a  crimson  and  an  emerald- 
green  appeal  most  strongly  to  the  eye. 

In   the   year    1864    Sir   William    Huggins   first 


The  Life  of  a  Star.  21 

applied  the  spectroscope  to  the  study  of  the  nebulas, 
the  particular  one  selected  being  a  small  but  com- 
paratively bright  object  in  the  constellation  of  the 
Dragon.  The  light  from  the  nebula  was  condensed 
upon  the  slit  of  the  spectroscope  by  the  object-glass, 
8  inches  in  diameter,  of  an  astronomical  tele- 
scope; and  at  the  first  glance,  the  examination  of 
the  spectrum  showed  it  to  be  characteristic  of  the 
light  emitted  from  a  glowing  gas,  since  it  consisted, 
not  of  a  continuous  band,  but  of  three  separated 
lines,  all  of  them  being  of  a  green  colour.  The 
luminous  matter  of  the  nebula  consisted,  therefore, 
not  of  a  host  of  stars,  but  of  incandescent  gas ;  and 
the  more  matured  views  of  Sir  William  Herschel 
were  established  upon  a  sound  scientific  basis. 

During  the  four  years  following  this  observation 
Huggins  subjected  the  light  from  seventy  other 
nebulae  to  analysis;  and  of  them  about  one-third, 
including  the  Great  Nebula  in  Orion,  proved  to  be 
gaseous.  The  remaining  two-thirds  yielded  "  con- 
tinuous" spectra,  spectra  in  which  all  shades  of 
colour  were  represented,  and  might,  therefore,  so 
far  as  spectroscopic  evidence  was  concerned,  con- 
sist of  systems  of  stars,  of  gas  possessing  compara- 
tively high  density,  or  of  gas  in  an  incipient  stage 
of  condensation.  The  structure  of  some  of  these  as 
revealed  by  the  photographic  plate  lends  strong 
support  to  the  last  hypothesis ;  in  the  Great  Nebula 
in  Andromeda,  for  instance,  it  is  scarcely  possible  not 
to  recognize  the  process  of  condensation  as  actually  in 
progress.  Nearly  one-half  of  the  nebulae  owe  their 
luminosity  to  the  presence  in  them  of  glowing  gas. 


22  Recent  Advances  in  Astronomy. 

It  is  difficult  not  to  see  in  the  gaseous  nebulas  the 
stuff  of  which  future  stars  will  be  made.  Granting 
that  their  substance  is  subject  to  the  law  of  gravita- 
tion, it  appears  certain  that  in  coming  ages  their 
glowing  matter  must,  under  its  influence,  be  drawn 
towards  centres  of  condensation;  the  smaller  and 
more  symmetrical  of  the  nebulas  possibly  developing 
into  single  stars,  but  such  majestic  collections  of 
cloudy  structures  as  are  revealed  in  Orion  being 
more  probably  the  origin  of  hosts  of  separate 
suns. 

Turning  from  these  impressive  representations  of 
the  birth  of  suns,  it  now  becomes  our  task  to  seek 
among  the  heavenly  bodies  for  the  more  sombre  but 
scarcely  less  impressive  record  of  their  death;  to 
search  among  their  resplendent  brethren  for  evidence 
of  the  existence  of  spent  and  dark  suns.  A  dark  star 
may  conceivably  become  known  to  us  in  either  of 
two  ways :  it  may  in  its  wanderings  through  space 
interpose  itself  between  the  Earth  and  a  bright  star, 
thus  producing  a  total  or  a  partial  eclipse  of  the 
latter ;  or  it  may  approach  sufficiently  near  a  visible 
star  to  affect  it  sensibly  by  its  gravitational  influence, 
in  which  case  it  may  be  possible  to  deduce  the  ex- 
istence of  the  dark  star  from  the  disturbance  apparent 
in  the  movement  of  the  bright  one.  There  can  be 
no  doubt  that  the  existence  of  dark  stars  has  been 
revealed  in  both  of  these  ways,  and  both  methods  of 
research  are  admirably  illustrated  in  the  discovery 
of  the  notorious  dark  companion  of  Algol. 

From  the  extremity  of  the  shallower  and  left  arm 
of  the  familiar  W  of  Cassiopeia,  and  setting  off  in 


The  Life  of  a  Star.  23 

a  direction  making  sensibly  a  right  angle  with  the 
limb,  a  gracefully  curved  line  is  naturally  traced  in 
the  heavens  by  the  stars  of  Perseus  and  terminated 
in  the  resplendent  orb  of  Capella.  A  straight  line 
diverging  to  the  left  of  this  stream  and  proceeding 
slightly  forwards  from  the  brightest  and  nearly 
central  star  in  Perseus,  is  directed  to  Algol,  the  best- 
known  of  the  variable  stars. 

The  variable  character  of  the  light  of  Algol  is  said 
to  have  been  first  observed  by  Montanari  in  1669, 
though,  owing  to  the  great  difficulty  in  measuring 
the  intensity  of  starlight,  it  has  only  been  possible 
in  recent  years  to  trace  the  exact  law  of  its  variation 
with  any  approach  to  scientific  accuracy.  Normally, 
Algol  appears  as  one  of  the  conspicuously  brilliant 
stars  of  the  heavens,  its  brightness  being  sensibly 
the  same  as  that  of  the  Pole  Star.  At  intervals  ot 
time  that  appear  to  be  subject  to  a  very  slow  varia- 
tion, and  which  are  at  present  represented  by  2  days 
10  hours  48  minutes  and  52  seconds,  its  light  com- 
mences to  fade,  and  continues  to  do  so  for  4 ]/2  hours, 
by  which  time  it  has  decreased  to  two-fifths  of  its 
normal  brightness.  This  minimum  value  it  retains 
for  20  minutes,  after  which  it  resumes  its  normal 
lustre  in  a  manner  which  is  nearly,  though  not 
exactly,  the  reversed  image  of  its  fading. 

In  1782  Goodricke,  impressed  with  the  regularity 
displayed  in  the  repeated  variations  of  the  star's 
light,  suggested  as  the  cause  of  it  the  existence  of  a 
dark  companion  revolving  round  Algol  in  an  orbit 
presented  edgeways  to  the  Earth,  so  that  at  each 
revolution  the  bright  star  would  suffer  partial  eclipse 


24  Recent  Advances  in  Astronomy. 

by  the  interposition  of  its  companion  between  it  and 
the  Earth.  The  explanation  was  obviously  sufficient 
to  account  for  the  mere  fact  of  periodic  variation,  and 
its  truth  appeared  more  probable,  when,  a  century 
later,  the  spectroscope  showed  the  variation  of  the 
star's  light  to  be  unaccompanied  by  any  change  in 
its  quality.  Such  change  would  indicate  change  in 
the  star  itself,  and  is  frequently  a  conspicuous  feature 
in  the  variation  of  other  and  less  regularly  variable 
stars.  The  probability  of  the  truth  of  the  eclipse 
theory  of  Algol  was  still  further  increased  when  in 
1888  Professor  E.  C.  Pickering  of  Harvard,  by  the 
application  of  the  "  meridian  photometer",  an  in- 
strument by  the  invention  of  which  it  became  possible 
to  measure  the  intensity  of  the  light  of  a  star  with  a 
degree  of  accuracy  previously  unattainable,  found, 
from  the  examination  of  the  light  of  Algol  at  repeated 
intervals  during  the  progress  of  its  variation,  the  law 
or  method  of  its  variation  to  be  essentially  such  as 
would  result  from  the  interposition  of  a  dark  sphere 
between  the  Earth  and  a  luminous  one. 

Closely  following  upon  Pickering's  researches, 
and  by  the  application  of  a  principle  suggested  by 
him,  the  final  demonstration  of  the  existence  of 
Algol's  dark  companion  was  effected  by  Vogel  at 
Potsdam,  from  observations  made  between  the  years 
1888  and  1891.  Assuming  the  existence  of  a  star 
revolving  round  Algol,  it  would  appear  probable 
that  the  force  necessary  to  constrain  it  to  continually 
follow  its  curved  path  would  be  found  in  gravitational 
attraction  exercised  upon  it  by  Algol.  By  such  a 
force,  the  attraction  of  the  Earth,  the  Moon  is  main- 


The  Life  of  a  Star.  25 

tained  in  its  nearly  circular  path  around  it;  by  such 
forces,  the  attractions  exercised  upon  them  by  the 
Sun,  the  planets  follow  without  deviation  their  de- 
termined orbits.  If,  however,  Algol  attracts  its 
companion,  it  follows  from  the  necessary  equality 
between  action  and  reaction  as  expressed  in  Newton's 
Third  Law  of  Motion,  that  the  companion  must 
attract  Algol  with  an  equal  and  opposite  force,  and 
it  is  conceivable  that  motion  of  Algol  caused  by  the 
attraction  of  the  companion  might  be  capable  of 
detection. 

The  character  of  the  motion  of  two  mutually  at- 
tracting bodies  was  first  determined  by  Newton.  It 
was  shown  by  him  to  follow  from  the  laws  of  motion, 
combined  with  the  fact  that  the  attraction  of  gravi- 
tation varies  inversely  with  the  square  of  the  distance 
separating  the  attracting  masses,  that  the  pair  must 
describe  similar  conic  sections  having  a  common 
focus,  which  is  continually  occupied  by  the  centre 
of  mass  of  the  pair.1  Which  of  the  three  possible 
forms  of  conic  section  will  be  assumed  by  the  orbits 
depends  upon  the  initial  circumstances  of  the  motion, 
but  the  greatest  interest  is  attached  to  the  ellipse, 
which,  being  the  only  conic  section  forming  a  closed 
curve,  must  be  the  orbit  in  every  case  in  which  the 
motion  is  repeated. 

The  mutual  revolution  of  the  Earth  and  Moon 
supplies  an  interesting  illustration  of  the  nature  of 
the  motion  under  consideration.  The  Moon  is 

1The  term  "centre  of  mass"  corresponds  to  the  point  more  generally 
known  in  elementary  mechanics  as  "centre  of  gravity".  For  obvious 
reasons  the  term  "  centre  of  gravity"  would  be  quite  inappropriate  in  cases 
similar  to  that  under  consideration. 


26  Recent  Advances  in  Astronomy. 

maintained  in  its  elliptical  and  nearly  circular  orbit 
by  the  gravitational  attraction  of  the  Earth.1  The 
Moon  must  therefore  attract  the  Earth  with  a  force 
equal  to  this;  and  the  Earth,  being  in  no  way 
anchored  in  space,  must  move  under  the  influence 
of  the  Moon's  attraction.  The  fact  of  its  motion  is 
beyond  doubt,  both  from  theoretical  considerations 
and  from  practical  observation ;  and  the  nature  of  it 
is  expressed  by  the  statement  that  the  Earth  and 
Moon  continually  describe  similar  elliptical  and 
nearly  circular  orbits  about  their  centre  of  mass,  this 
point  being  in  a  common  focus  and  nearly  in  the 
centre  of  each  orbit.  The  general  statement  that 
the  Moon  describes  an  elliptical  orbit  round  the 
Earth  is,  therefore,  though  not  inexact,  incomplete. 
It  would  be  equally  true,  and  not  inexact,  to  regard 
the  Earth  as  describing  an  elliptical  orbit  round  the 
Moon.  Since,  however,  the  mass  of  the  Earth  is 
eighty  times  that  of  the  Moon,  the  centre  of  mass  of 
the  pair  is  eighty  times  nearer  to  the  centre  of  the 
Earth  than  to  the  centre  of  the  Moon,  lying  in  con- 
sequence well  within  the  Earth  itself;  so  that  the 
actual  orbit  described  by  the  Moon  is  far  larger  than 
that  described  by  the  Earth.  It  is  the  common 
centre  of  mass  of  the  Earth  and  Moon  that  describes 
an  elliptical  orbit  yearly  about  the  Sun. 

With  the  assistance  of  the  diagram  given  in  fig.  2 
there  will  be  no  difficulty  in  forming  a  definite  picture 
of  the  system  of  Algol  and  its  companion,  and  of 

1  The  reader  may  be  reminded  that  the  circle  is  merely  a  particular  form 
of  an  ellipse,  that  in  which  the  greatest  and  least  lines  drawn  through  the 
centre,  or  the  major  and  minor  axes,  are  of  equal  length.  The  focus  of  a 
circle  and  its  centre  coincide. 


The  Life  of  a  Star.  27 

their  relative  movements.  The  point  o  is  the  centre 
of  mass  of  the  pair,  and  since  it  is  represented  as 
one-half  as  far  from  Algol  as  from  the  companion, 
the  companion  is  regarded  as  possessing  one-half 
the  mass  of  Algol. 
The  orbits  are  re- 
presented as  circles, 
though,  in  accord- 
ance with  the  law 
of  gravitation,  they 
might  be  any  va- 
riety of  similar 
ellipses.  Whatever 
the  relative  masses  Fig  2  _The  System  of  AlgoL 

of    the     pair,    and 

whatever  the  degree  of  ellipticity  of  the  orbits,  the 
general  principle,  however,  remains  unaffected.  The 
Solar  System  is  imagined  as  lying  far  away  upon 
the  right;  and,  from  the  fact  that  no  parallax  has 
been  detected  in  Algol,  it  follows  that  upon  the  scale 
according  to  which  the  orbit  of  Algol  is  represented, 
the  distance  of  the  Solar  System  must  be,  at  the 
least  estimate,  5  miles.  When  at  A,  Algol  will  be 
eclipsed  by  the  interposition  between  it  and  the 
Earth  of  its  dark  companion  at  c.  From  these 
positions  the  star  and  its  companion  will  proceed  in 
their  orbital  revolutions,  moving  in  the  directions 
indicated  by  the  arrows,  their  relative  speeds  being 
determined  by  the  condition  that  the  pair  must  at 
every  instant  lie  upon  opposite  sides  of  the  centre  of 
mass,  the  position  of  which  remains  unaffected  by 
their  motion.  It  will  be  clear,  therefore,  that  if  the 


28  Recent  Advances  in  Astronomy. 

hypothesis  of  the  dark  star's  existence  is  sound,  to 
an  observer  upon  the  Earth  provided  with  sufficiently 
delicate  means  of  observation,  Algol  should  appear 
to  swing  to  and  fro  about  the  point  o,  attaining  its 
greatest  displacement  upon  either  side  of  it  when  at 
A'  and  A''.  If,  however,  the  orbit  of  Algol  were  even 
to  equal  that  of  the  Earth  round  the  sun  in  magni- 
tude, the  consequent  displacement  in  its  position 
would  be  so  slight  as  to  escape  detection  by  the 
most  refined  observational  means  existing,1  and  it 
has,  in  fact,  never  been  detected. 

During  the  orbital  revolution  of  Algol  there  is, 
however,  a  relative  displacement  of  another  kind 
between  it  and  the  Earth.  In  executing  one  half 
of  its  orbit  the  star  must  continually  approach  the 
Earth,  while  during  the  other  half  it  must  recede 
from  it.  Assuming  the  orbits  to  be  circles,  the 
star  should  approach  the  Earth  in  moving  from 
the  position  A  in  which  it  is  eclipsed,  the  approach 
becoming  direct,  and  therefore  most  rapid  at  A', 
a  quarter  period  later;  while  at  A"  there  should 
exist  an  equally  rapid  and  direct  motion  of  recession. 
It  is  in  the  detection  of  these  alternate  movements 
of  approach  and  recession  that  Vogel  has  achieved 
one  of  the  most  remarkable  triumphs  of  observa- 
tional astronomy. 

The  immediate  principle,  to  the  successful  appli- 
cation of  which  Vogel's  demonstration  of  the 
motion  of  Algol  is  due,  will  be  more  fully  explained 
in  a  later  chapter.  It  follows  as  a  necessary  con- 

i  This  conclusion  is  directly  involved  in  the  statement  that  the  parallax  of 
Algol  is  inappreciable  by  the  most  refined  observational  means  existing. 


The  Life  of  a  Star.  29 

sequence  of  the  wave  theory  of  light  that  a  source 
of  light  approaching  the  observer  should  crowd 
together  and  thus  shorten  its  light-waves  in  front 
of  it,  and  in  consequence  alter  the  nature  of  the 
light,  raising  its  colour  in  the  spectral  series,  that 
is,  causing  it  to  approach  the  violet  in  hue,  and 
increasing  its  refrangibility.  A  movement  of  re- 
cession should  correspondingly  draw  out,  and  thus 
lengthen,  the  light -waves  travelling  behind  the 
source  and  towards  the  observer,  lowering  the 
colour  towards  the  red  of  the  spectrum,  and  de- 
creasing the  refrangibility.  In  1842  Doppler  had 
suggested  that  the  apparent  colours  of  certain  stars 
might  thus  be  affected  by  their  movement,  a  rapidly 
approaching  star  acquiring  a  bluish,  and  a  rapidly 
receding  one  a  ruddy  tinge.  The  suggestion, 
however,  failed,  for  various  reasons;  among  them, 
the  fatal  one,  that,  owing  to  the  high  speed  of  light, 
such  transcendent  velocities  as  would  be  necessary 
to  produce  such  a  change  in  the  colour  of  a  star 
that  should  be  appreciable  to  the  eye  would  trans- 
form the  whole  aspect  of  the  heavens  in  a  few  years. 
The  true  direction  in  which  to  search  for  a  record 
in  the  light  of  a  star  of  indications  of  its  approach 
or  recession  was  indicated  by  Fizeau  in  1848,  and 
lies  in  the  careful  measurement  of  the  positions  of 
the  dark  lines  with  which  the  spectra  of  the  Sun 
and  of  the  greater  number  of  the  stars  are  ruled 
throughout,  the  suggestiveness  of  which  was  at  that 
time  beginning  to  be  recognized.  These  dark  lines 
simply  indicate  colours  absent  in  sunlight  and  star- 
light, and  the  absent  colours  are  affected  by  the 


30  Recent  Advances  in  Astronomy. 

motion  of  the  source  precisely  as  are  those  actually 
present.  Consequently  the  approach  of  a  star 
should  raise  the  colours  absent  in  its  light  towards 
the  violet,  and  the  dark  lines  in  its  spectrum  should 
therefore  be  displaced  toward  the  violet  end  of  the 
spectrum;  the  reverse  occurring  in  the  case  of  a 
receding  star.  In  1868  Sir  William  Huggins 
succeeded  in  detecting  slight  displacements  in  the 
spectral  lines  of  certain  stars,  and,  assigning  the 
displacements  to  this  cause,  estimated  from  them 
the  velocities  of  the  stars  in  the  direction  of  the 
line  of  sight.  It  was  this  method  that  Picker- 
ing suggested  should  be  brought  to  bear  upon 
the  problem  of  Algol  and  its  hypothetical  com- 
panion. 

In  photographs  of  the  spectrum  of  Algol,  taken 
at  intervals  during  the  years  1888  to  1891,  the 
movement  of  the  star,  and  precisely  such  movement 
as  was  demanded  by  the  eclipse  theory,  was  estab- 
lished beyond  doubt.  When  under  eclipse,  as 
well  as  later  by  an  interval  equal  to  one-half  of  that 
between  successive  eclipses,  at  which  time  the  star 
should  be  between  its  companion  and  the  Earth, 
the  spectrum  of  its  light  should  be  normal,  since 
at  these  instants  its  motion  should  be  directly  across 
the  line  of  sight,  and  should  neither  be  towards  nor 
from  the  observer.  During  the  half-period  preced- 
ing eclipse  the  motion  of  the  star  should  be  from 
the  observer,  and  the  spectral  lines  should  be  there- 
fore displaced  towards  the  red  as  the  result  of  the 
drawing  out  of  the  light-waves,  while  after  eclipse 
the  motion  of  recession  should  be  replaced  by  one 


The  Life  of  a  Star.  31 

of  approach,  and  the  spectral  lines  should  be  shifted 
towards  the  violet.  Every  one  of  these  predictions 
was  confirmed  in  Vogel's  photographs.  The  maxi- 
mum displacement  of  the  lines,  which  occurred,  as 
they  should  have,  at  quarter-periods  before  and  after 
eclipse,  indicated  velocities  of  recession  of  24-4, 
and  approach  of  28-6  miles  per  second  respectively, 
the  difference  between  the  two  values  being  natu- 
rally explained  upon  the  assumption  that  the  speed 
of  Algol  in  its  orbit  is  26-5  miles  per  second,  the 
mean  of  the  two,  and  that  the  system  of  Algol  and 
the  companion  is  approaching  the  Solar  System 
with  a  speed  of  2*1  miles  per  second.  Knowing  the 
orbital  speed  of  Algol,  as  well  as  its  period  of  re- 
volution,— the  interval  between  successive  eclipses, 
— it  is  a  simple  matter  to  calculate  the  circumference, 
and  from  it  the  radius,  of  its  orbit.  The  final  result 
is  almost  exactly  a  million  miles,  from  which  it 
follows  that  the  oscillation  of  Algol  across  the  line 
of  sight  is  far  too  small  to  be  capable  of  detection. 

The  dark  companion  of  Algol  suggests  the  picture 
of  the  death-stage  of  a  sun.  In  nine  other  stars, 
variation  in  light,  in  nature  similar  to  that  exhibited 
by  Algol,  points  strongly  to  a  similar  cause.  In 
another  star,  Spica,  the  existence  of  an  invisible 
companion  is  indicated  by  the  displacement  of 
spectral  lines,  though  no  eclipse  results,  probably 
from  the  plane  of  the  orbits  making  a  sufficiently 
large  angle  with  the  line  of  sight  for  the  dark  star 
to  clear  the  bright  one  at  each  revolution. 

In  the  few  cases  in  which  the  existence  of  dark  stars 
has  been  revealed,  their  detection  has  been  due  to 


32  Recent  Advances  in  Astronomy. 

the  fact  of  their  close  association  with  bright  stars. 
Only  by  an  inconceivably  remote  chance  would  it 
be  possible  to  become  aware  of  the  existence  of  an 
isolated  dark  star  by  either  of  the  methods  that 
have  been  so  successfully  applied  to  the  companion 
of  Algol.  Such  knowledge  of  stellar  distances  as 
we  possess  renders  it  probable  that  the  suns  of 
space  are  separated  from  their  nearest  neighbours 
by  depths  so  vast  that  were  there  dark  stars  scat- 
tered at  random  among  them  exceeding  the  bright 
ones  by  many  times  in  number,  the  probability  of 
one  of  them  approaching  so  near  to  a  visible  star 
as  to  sensibly  affect  it  by  gravitation  would  be 
excessively  remote,1  while,  since  it  is  not  possible 
to  continually  examine  more  than  an  insignificant 
minority  of  the  visible  stars,  either  as  regards 
position  upon  the  face  of  the  sky,  by  change  of 
which  motion  across  the  line  of  sight  would  be 
apparent,  or  spectroscopically,  by  which  motion  in 
the  line  of  sight  might  be  revealed,  it  is  probable 
that  millions  of  near  approaches  between  isolated 
dark  stars  and  brilliant  ones  would  occur  before  the 
effect  of  one  would  be  detected. 

The  probability  of  becoming  aware  of  the  exist- 
ence of  a  dark  star  by  its  drifting  by  chance  across 
the  line  of  sight  directed  from  the  Earth  toward  a 
more  distant  brilliant  one  appears  equally  remote. 
It  is  impossible  to  contemplate  even  the  most 
crowded  regions  of  the  heavens  through  a  telescope 
of  fine  quality,  and  of  large  light-grasping  power, 

1  With  the  exception  possibly  of  the  more  crowded  regions  of  the  Milky 
Way. 


The  Life  of  a  Star.  33 

without  recognizing  that  among  the  myriads  of 
bright  points  scattered  over  the  field  of  view  there 
is  ample  room  for  the  existence  of  dark  stars  far 
exceeding  them  in  number.  The  brighter  only 
among  the  stars  appear  in  the  telescope  as  discs 
of  sensible  dimensions ;  but  the  reader  is  probably 
aware  that  such  "  spurious"  discs  result  from  imper- 
fections in  the  eye,  and  from  the  inherent  principles 
of  telescopic  construction.  Were  the  telescope 
and  the  eye  alike  perfect,  such  is  the  stupendous 
remoteness  of  the  stars,  that,  although  suns,  the 
nearest  of  them,  even  if  far  exceeding  our  Sun  in 
magnitude,  would  appear  under  the  highest  magni- 
fying power  that  has  so  far  been  applied  to  them,  as 
mere  specks  of  light  devoid  of  sensible  dimension. 

Although  the  apparent  dimensions  of  stars  are 
far  beyond  the  possibility  of  detection  with  the 
most  perfect  optical  means,  it  is,  however,  possible 
to  make  a  rough  estimate  of  the  extent  of  sky 
covered  by  individual  stars  or  by  the  whole  collec- 
tion. The  possibility  of  effecting  this  is  based 
upon  the  fact  that  the  apparent  brightness  of  any 
surface  is  independent  of  its  remoteness.  If,  for 
instance,  the  Sun  were  removed  to  three  times  its 
present  distance,  the  light  received  from  it  would, 
according  to  the  law  of  inverse  squares,  be  reduced 
to  one-ninth  of  its  present  value;  but  since  its 
apparent  size — or  the  area  in  the  sky  covered  by 
it — would  similarly  be  reduced  to  one -ninth,  the 
apparent  brightness  of  its  surface  would  remain 
unchanged.  If,  therefore,  there  were  a  number 
of  sun-like  bodies  in  space,  each  of  the  same  in- 

(M520)  C 


34  Recent  Advances  in  Astronomy. 

trinsic  brilliancy  as  the  Sun,  but  differing  both  in 
size  and  distance  from  the  Earth,  it  would  follow 
that,  since  the  apparent  surface  brightness  of  all 
would  be  the  same,  the  amount  of  light  received 
from  any  one  of  them  would  be  in  direct  proportion 
to  its  apparent  size,  or  to  the  sky-surface  covered 
by  it. 

The  total  amount  of  light  received  from  all  the 
stars  above  '9^  magnitude  (visibility  to  the  naked 
eye  terminates  at  the  6th  magnitude)  has  been  esti- 
mated by  Mr.  Plummer.  The  result,  slightly 
modified  in  accordance  with  more  recent  measure- 
ments of  the  brightness  of  Sirius,  far  the  most 
important  star  of  the  whole,  is  given  by  Miss  Clerke 
as  one-eightieth  of  that  of  the  Full  Moon.  The  ratio 
of  the  light  of  the  Sun  to  that  of  the  Full  Moon 
has  been  estimated  by  Zollner  as  619,000  to  i,  so 
that  the  Sun  exceeds  the  total  of  the  stars  above 
9^  magnitude  in  their  illumination  of  the  Earth 
by  80  times  619,000,  or  nearly  50  million  times. 
If,  therefore,  the  assumption  be  made  that  the 
surfaces  of  the  stars  are  of  the  same  intrinsic 
brilliancy  as  the  Sun,  it  follows  that  the  stars  cover 
a  portion  of  the  sky  equal  to  one  5o-millionth  of 
that  covered  by  the  Sun ;  and  since  the  Sun  covers 
but  one  2io,oooth  of  the  total  of  the  sky,  it  follows 
that  the  stars  would  cover  rather  less  than  one  10- 
billionth. 

It  was  necessary  to  limit  the  above  estimate  to 
the  324,000  stars  above  the  9^  magnitude  as  there 
is  no  means  of  determining  the  light  received  from 
the  fainter  ones.  These  324,000  stars,  however,  far 


The  Life  of  a  Star.  35 

more  than  include  those  among  which  there  could 
be  any  hope  of  detecting  an  eclipse  by  an  isolated 
dark  star. 

Should  dark  stars  far  exceeding  the  bright  ones 
in  number  exist  in  celestial  space,  an  eclipse  of  one 
of  the  latter  would  therefore  be  a  phenomenon  of 
rare  occurrence ;  while,  should  an  eclipse  occur,  so 
few  are  the  stars  the  brightness  of  which  is  sub- 
jected to  continual  scrutiny  that  the  probability  of 
its  passing  unnoticed  is  overwhelming. 

The  possibility  of  an  unseen  system  of  stars  per- 
meating the  seen  is  beyond  doubt.  The  system  of 
the  seen  is  indeed  sufficient  to  satisfy  the  highest 
ambition  of  imagination,  but  he  would  be  bold  who 
should  assert  that  it  may  not  well  form  but  an 
insignificant  fraction  of  a  still  more  surpassingly 
transcendent  whole. 

The  question  as  to  the  nature  of  the  changes  now 
taking  place  in  the  Sun  is  one  of  very  great  interest, 
and  its  study  involves  physical  considerations  of 
high  importance  and  not  free  from  grave  difficulty. 

The  delicate  and  mottled  tracery  visible  under 
the  most  favourable  conditions  for  telescopic  obser- 
vation over  the  entire  surface  of  the  Sun,  is  strongly 
suggestive  of  the  view  that  its  bright  surface — 
generally  known  as  the  photosphere — consists  of 
an  accumulation  of  incandescent  clouds.  Such  a 
cloudy  structure  of  the  photosphere  is  in  harmony 
with  the  general  results  of  solar  observation,  more 
especially,  perhaps,  with  the  nature  and  the  rapidity 
of  the  changes  frequently  characteristic  of  sun  spots, 
which,  according  to  this  view,  are  either  depressions 


36  Recent  Advances  in  Astronomy. 

or  actual  gaps  in  the  photosphere.  The  spectro- 
scope indicates  as  existing  above  the  level  of  the 
photosphere  a  solar  atmosphere,  as  constituents  of 
which  the  vapours  of  hydrogen,  calcium,  iron,  and 
other  metals  are  conspicuous,  and  in  which  are 
traceable  with  greater  difficulty  those  of  a  few  of 
the  non- metallic  elements.  It  is  not  difficult  to 
imagine  the  process  of  formation  of  the  clouds  of 
the  photosphere  from  the  precipitation  as  fog  of  the 
more  readily  condensable  of  these  vapours  by  their 
cooling  consequent  upon  their  being  carried  into 
the  upper  regions  of  the  atmosphere. 

The  question  as  to  the  physical  conditions  exist- 
ing in  the  interior  of  the  Sun  is  attended  with 
graver  difficulty,  and  is  of  the  first  importance  in 
the  problem  under  consideration.  Herschel  im- 
agined as  existing  beneath  the  clouds  of  the  photo- 
sphere, a  solid  globe ;  and  even  advanced  the  view, 
so  preposterous  to  modern  students  of  physical 
science,  that  it  might,  from  the  protection  of  a 
second  and  intervening  cloud  shell  cool  and  im- 
pervious to  heat  radiation,  be  protected  from  the 
intense  glare  of  the  photosphere  above  to  such  an 
extent  as  to  be  a  cool  and  habitable  world.  When 
the  necessity  for  the  interior  heat  of  the  Sun  being 
at  least  as  high  as  that  of  its  exterior  became  recog- 
nized, the  solid  globe  was  generally  replaced  by  an 
ocean  of  molten  matter. 

It  is,  however,  scarcely  possible  to  regard  as 
existing  in  the  interior  of  the  Sun,  matter  in  either 
the  solid  or  in  the  liquid  condition.  The  tempera- 
ture above  the  photosphere  is  such  that  iron,  car- 


The  Life  of  a  Star.  37 

bon,  and  other  among  the  most  refractory  of  ele- 
ments known  to  terrestrial  chemistry  are  found  in  it 
in  the  gaseous  state ;  and  the  temperature  of  these 
external  regions  must  be  far  lower  than  that  of  the 
interior.  It  was  for  a  time  regarded  as  barely  pos- 
sible that  the  enormous  pressure  that  must  exist  at 
great  depths  in  the  interior  of  the  Sun  might  be 
effective  in  maintaining  matter  in  the  solid  or  liquid 
condition  in  spite  of  the  high  temperature,  since  it 
is  a  familiar  fact  in  laboratory  experience,  that  lique- 
faction of  a  gas  is  in  every  case  assisted  by  pressure, 
and  may  in  many  instances  apparently  be  effected  by 
it  alone.  Since,  however,  it  became  apparent  from 
the  classical  researches  of  Dr.  Andrews  in  1869, 
that  there  exists  for  every  element  a  critical  tempera- 
ture, above  which  it  is  impossible  for  it  under  any 
conditions  of  pressure  to  assume  the  liquid  state,  it 
has  generally  been  regarded  that  a  liquid  interior 
to  the  Sun  is  next  to  an  impossibility.  The  Sun  is, 
in  all  probability,  essentially  an  enormous  bubble, 
enveloped  in  incandescent  cloud,  from  which,  by 
the  mechanism  of  radiation,  its  energy  is  transmitted 
into  external  space. 

From  the  fact  that  the  degradation  of  a  star  from 
its  earliest  nebular  to  its  dark  state  is  the  direct 
consequence  of  the  radiation  of  its  heat  into  space ; 
and  as,  in  ordinary  experience,  loss  of  heat  is  ac- 
companied by  fall  in  temperature,  it  has  frequently 
been  assumed  that  the  life  of  a  star  must  be  the 
record  of  continual  fall  in  temperature ;  and  that  in 
the  nebulae  would  be  found  the  highest  temperatures 
represented  in  celestial  bodies.  To  the  " fiery  mist" 


38  Recent  Advances  in  Astronomy. 

from  which  Laplace,  in  1796,  had  imagined  the 
development  of  the  system  of  the  Sun  and  planets, 
a  temperature  was  assigned  far  higher  than  that  of 
the  Sun  at  present ;  and  the  same  view  was  extended 
to  the  nebulae,  when  the  demonstration  of  their 
gaseous  nature  had  indicated  them  as  fulfilling  in 
the  Cosmos  the  functions  of  embryonic  stars. 

Recent  considerations  based  upon  the  experimen- 
tally ascertained  properties  of  gases  and  upon  the 
principle  of  conservation  of  energy  have,  however, 
shown  that  this  simple  view  cannot  be  maintained. 
Attention  has  already  been  directed  to  the  fact  that 
each  step  in  the  radiation  of  heat  from  the  Sun  brings 
about  a  shrinkage  of  its  bulk,  or,  more  exactly, 
enables  the  gravitation  of  its  parts  to  draw  them 
closer  together,  and  that  by  this  act  of  compression 
other  heat  is  developed.  In  a  very  remarkable 
paper,  published  in  1870,  Mr.  Homer  Lane  has 
shown  that  if  the  Sun  were  entirely  gaseous,  and  if 
the  gases  composing  it  were  under  such  physical 
conditions  that  the  laws  of  "  perfect  gases"  should 
be  applicable  to  them,  the  heat  developed  by  shrink- 
age must  not  merely  equal  but  must  so  far  exceed 
that  radiated  to  effect  it,  that  the  temperature  of  the 
whole  must  actually  rise  in  consequence,  and  must 
continue  to  do  so  for  so  long  as  a  perfectly  gaseous 
condition  is  maintained. 

A  " perfect  gas"  is  defined  as  one  in  which,  for 
so  long  as  its  temperature  is  unchanged,  any  increase 
in  pressure  brings  about  a  proportionate  decrease 
in  volume.  This  condition,  known  as  "  Boyle's 
Law  ",  is  very  closely  fulfilled  by  hydrogen,  oxygen, 


The  Life  of  a  Star.  39 

and  nitrogen,  as  well  as  by  gases  in  general  when 
under  conditions  far  removed  from  those  under 
which  they  assume  the  liquid  condition,  so  long  as 
their  density  is  not  rendered  excessive  by  intense 
pressure.  Under  extreme  pressure,  however,  de- 
crease in  volume  becomes  increasingly  less  than 
that  demanded  in  Boyle's  Law,  and  it  is  probable 
that  for  every  gas  at  a  given  temperature  there  is  a 
limiting  volume  beyond  which  it  cannot  be  com- 
pressed by  any  pressure  however  great. 

The  statement  of  Lane's  theorem — that  it  is  pos- 
sible under  certain  conditions  for  a  body  to  rise  in 
temperature  as  the  result  of  its  loss  of  heat — appears 
at  first  so  contrary  to  common  experience  that  there 
is  generally  great  difficulty  in  thoroughly  accepting 
it.  That  emission  of  heat  is  not  inseparably  asso- 
ciated with  fall  of  temperature,  will,  however,  be 
clear  from  the  consideration  of  such  instances  as  are 
supplied  by  the  condensation  of  a  vapour  and  the 
solidification  of  a  liquid.  The  passage  of  steam 
into  water  at  its  boiling-point  is  unaccompanied  by 
any  fall  in  temperature,  though  the  amount  of  heat 
given  out  is  more  than  five  times  that  necessary  to 
raise  the  temperature  of  the  water  from  its  freezing- 
point  to  its  boiling-point.  Similarly,  the  freezing  of 
water  is  unaccompanied  by  any  fall  in  temperature, 
though  here  again  a  large  amount  of  heat  is  emitted 
by  the  solidifying  water — four-fifths  of  that  required 
to  raise  the  water  from  its  freezing-  to  its  boiling- 
point.  In  a  mass  of  gas  subject  to  no  external  force, 
development  of  heat  results  from  its  compression 
under  forces  due  to  the  gravitation  of  its  parts;  it  is 


40  Recent  Advances  in  Astronomy. 

loss  of  heat,  not  fall  in  temperature,  that  enables  the 
gravitational  forces  to  become  effective  in  producing 
compression. 

In  the  hope  of  assisting  the  reader  towards  form- 
ing a  clear  picture  of  one  of  the  most  remarkable  of 
natural  processes,  a  confessedly  incomplete  demon- 
stration of  Lane's  theorem  is  given  in  the  following 
paragraph,  which  may  be  omitted  if  the  mechanical 
and  geometrical  principles  involved  should  not 
appear  sufficiently  familiar.  An  essentially  similar 
demonstration,  by  which  indeed  the  one  given  here 
was  suggested,  is  given  in  Newcomb's  Astronomy. 

Let  a  globe  of  a  "perfect"  gas  be  imagined, 
temperature  being  uniform  throughout  it,  and  let 
the  whole  be  at  rest,  free  from  internal  currents,  and 
subject  only  to  its  own  gravitation.  All  portions  of 
the  globe  are  attracted  toward  the  centre,  and  a 
pressure  is  produced  thereby  that  continually  in- 
creases toward  the  centre.  According  to  Boyle's 
Law  the  density  must  similarly  increase  toward  the 
centre.  Let  the  whole  globe  be  imagined  as  con- 
sisting of  a  number  of  concentric  spherical  shells, 
each  enveloping  those  within  it  in  the  manner  sug- 
gested by  the  coats  of  an  onion,  and  let  attention  be 
directed  to  one  of  these  shells.  The  total  pressure 
of  the  gas  comprising  the  shell  is  due  to  the  weight 
— that  is,  the  gravitation  toward  the  centre — of  the 
portion  of  the  gas  outside  of  it,  and  this  pressure  is 
distributed  over  the  outer  surface  of  the  shell.  Now 
imagine  the  globe  to  lose  heat  by  radiation,  and  to 
shrink  in  consequence  until  its  radius  has  become 
reduced  by  one-half.  If  the  process  occurs  so 


The  Life  of  a  Star.  41 

gradually  that  the  temperature  changes  uniformly 
throughout  the  whole,  all  portions  will  shrink 
equally,  the  radius  of  the  shell  will  be  reduced  to 
one-half,  and  therefore,  by  elementary  geometry, 
its  surface  will  be  one-fourth  and  its  volume  one- 
eighth  of  their  former  values.  The  distance  of 
every  part  of  the  globe  from  the  centre  will  be 
halved  and  the  attraction  of  each  portion  to  the 
centre  will,  since  gravitation  is  inversely  propor- 
tional to  the  square  of  the  distance,  therefore  be 
increased  fourfold.  The  whole  weight  of  the  portion 
of  the  globe  that  lies  beyond  the  shell  will  therefore 
be  increased  fourfold,  but,  as  this  weight  is  now 
distributed  over  one-fourth  of  the  former  surface,  the 
intensity  of  the  pressure  will  be  increased  to  sixteen 
times  its  former  value.  Such  an  increase  in  the 
intensity  of  the  pressure  would,  if  the  temperature 
of  the  shell  had  remained  unchanged,  compress  the 
gas  in  it  to  one-sixteenth  of  its  former  value.  It 
has,  however,  been  shown  that  the  gas  occupies 
one-eighth  of  its  former  volume, — double  the  volume 
that  it  should  occupy  had  the  temperature  remained 
unchanged.  Such  an  excess  in  volume  can  only 
be  due  to  increased  temperature,  and  its  tempera- 
ture must  consequently  have  risen. 

It  is  scarcely  necessary  to  add  that  a  shrinkage  of 
the  radius  to  one-half  is  assumed  only  for  the  sake 
of  simplicity;  the  same  result — a  necessary  rise  in 
temperature — would  follow  from  the  assumption  of 
any  given  contraction. 

If,  then,  the  Sun  behaves  as  a  perfect  gas,  its 
temperature  must  be  increasing  as  the  indirect 


42  Recent  Advances  in  Astronomy. 

result  of  the  torrent  of  heat  continually  radiated 
into  external  space.  There  is  good  reason  to  re- 
gard it  as  probable  that  the  Sun  is  in  the  main 
gaseous,  but  it  would  be  rash  to  assume  that  under 
the  extreme  conditions  of  pressure  and  temperature 
existing  in  its  interior,  the  laws  of  perfect  gases  are 
fulfilled  by  it  even  approximately.  The  properties 
of  gases  become  markedly  modified  even  at  such 
moderately  high  temperatures  and  pressures  as  it  is 
possible  to  produce  in  the  laboratory,  and  in  such  a 
manner  as  to  suggest  that  could  the  matter  of  the 
interior  of  the  Sun  be  subjected  to  examination, 
although  it  would  prove  to  be  neither  solid  nor 
liquid,  it  would  be  difficult  to  trace  in  it  the  gaseous 
characteristics  with  which  we  are  familiar.  It  is 
therefore  impossible  to  decide  the  interesting  point 
whether  the  Sun  is  at  present  rising  or  falling  in 
temperature,  though  there  can  be  little  doubt  that 
in  the  remote  past,  when  far  more  tenuous,  its  tem- 
perature must  have  been  lower  than  it  is  at  the 
present  time. 

Whatever  the  present  trend  of  the  temperature 
of  the  Sun,  it  is,  to  say  the  least,  unnecessary  to 
assume  a  high  temperature  for  the  nebula  from 
which  it  has  been  derived.  Imagining  a  nebula 
from  which  a  single  star  is  to  be  evolved  as  a  com- 
paratively cool  diffuse  extension  of  gas,  of  so  low  a 
temperature  and  of  so  great  a  tenuity  that  it  should 
obey  the  laws  of  perfect  gases,  not  necessarily  suffi- 
ciently hot  for  the  whole  of  its  constituents  to  exist 
in  it  in  the  gaseous  condition,  but  possibly  embrac- 
ing them  in  its  volume  as  discrete  solid  or  liquid 


The  Life  of  a  Star.  43 

particles,  it  becomes  possible  to  take  a  rough  fore- 
cast of  its  future  career.  Under  the  influence  of 
the  gravitational  attraction  upon  each  other  of  all 
its  parts,  it  would  tend  to  acquire  a  spherical  form. 
Heat  would  pass  into  space  by  radiation ;  gravita- 
tion would  in  consequence  be  enabled  to  draw  its 
parts  closer  together;  the  temperature  would  rise, 
and  portions  of  its  solid  or  liquid  ingredients  would 
become  gas.1  The  process  would  continue,  and 
after  a  time  the  nebula  would  become  in  the  main 
gaseous.  At  some  period  excessive  local  cooling 
in  the  outermost  parts  would  cause  condensation 
there  and  a  photospheric  cloud  shell  would  be 
formed.  The  nebula  has  now  become  a  sun.  For 
a  time  its  temperature  continues  to  rise  and  its 
radiation  becomes  more  and  more  effective.  At 
length,  however,  possibly  owing  to  excessive  con- 
densation in  the  photosphere,  possibly  to  tempera- 
ture and  density  increasing  in  the  interior  to  such 
an  extent  that  the  gaseous  laws  are  widely  trans- 
gressed, the  rise  in  temperature  ceases  and  is  soon 
replaced  by  a  fall.  The  Sun  has  passed  the  zenith 
of  its  career  and  is  now  descending  towards  extinc- 
tion ;  a  few  more  ages  and  its  radiant  activity  has 
ceased  to  be. 

The  question  whether  any  evidence  is  supplied 
by  stars  as  to  the  course  they  have  run  from  their 

xNo  doubt  heat  would  also  be  developed  from  collisions  between  the 
non-gaseous  constituents  of  the  nebula,  since  these  would  be  in  motion 
under  the  influence  of  gravitation.  It  might  even  be  that  the  first  evidence 
of  luminosity  in  the  whole  might  be  due  to  the  generation  of  heat  by  these 
collisions,  the  gas  itself  being  generally  below  the  temperature  of  incandes- 
cence, as  is  suggested  in  Sir  Norman  Lockyer's  Meteoritic  Hvfothesis. 


44  Recent  Advances  in  Astronomy. 

nebulous  condition — whether  among  those  visible  it 
is  possible  to  recognize  individuals  in  the  early 
period  of  their  career,  others  in  the  meridian  of 
their  glory,  and  others  again  upon  the  descending 
path  towards  extinction — is  among  the  most  fascinat- 
ing of  the  speculations  of  modern  astronomy.  It  is 
generally  regarded  that  such  evidence  is  indicated 
in  the  spectroscopic  analysis  of  their  light,  but  it 
must  be  confessed  that  this  branch  of  scientific 
inquiry  can  hardly  as  yet  be  regarded  as  having 
passed  beyond  the  speculative  stage.  From  it  we 
may  hope,  perhaps,  in  the  future,  to  be  able  to 
decide  whether  our  Sun  is  increasing  in  splendour, 
or  whether  he  has  passed  the  period  of  his  greatest 
glory.  Here  it  may  be  permissible  to  add  that,  in 
the  judgment  of  the  writer,  the  evidence,  though 
not  free  from  serious  difficulty  in  its  interpretation, 
appears  to  indicate  the  former  as  the  more  probably 
true  hypothesis,  and  that  in  the  remote  future  it  is 
not  inconceivable  that  radiations  of  the  Sun  should 
rival  even  those  of  Sirius  at  the  present  time. 
Should  this  be  so,  the  maximum  of  vitality  of  a 
star  must  be  thrown  far  forward  in  its  life  history, 
and  the  duration  of  its  decay  must  be  correspond- 
ingly brief. 

The  physical  universe  is  inexpressibly  glorious; 
and  it  is  scarcely  possible  that  the  contemplation  of 
the  decay  of  its  activity  should  be  unaccompanied 
by  a  touch  of  sadness.  One  is,  therefore,  led  to 
inquire,  whether  among  the  processes  of  nature  no 
means  are  indicated  by  which  its  lost  energy  may 
be  restored  to  a  dead  star.  So  far  as  the  working 


The  Life  of  a  Star.  45 

of  nature  is  revealed  in  the  laws  of  physical  science, 
the  only  way  in  which  a  star  can  re-assume  its 
nebulous  condition  is  by  a  collision  between  it  and 
another,  by  which  encounter  the  whole  or  part  of 
the  total  energy  of  motion  of  the  pair  would  be 
transformed  into  heat.  The  establishment  of  the 
equivalence  between  heat  and  motion,  one  of  the 
noblest  achievements  of  modern  science,  is  now  a 
familiar  fact  to  everyone.  By  the  destruction  of 
motion  heat  is  generated;  the  amount  of  heat  is 
directly  related  to  the  masses  and  velocities  of  the 
moving  matter  and  can  be  readily  calculated  from 
them ;  while,  in  its  turn,  the  heat  itself  may  under 
suitable  conditions  disappear,  and  in  so  doing  re- 
generate motion  identical  in  amount  with  the  quan- 
tity that  passed  out  of  existence  in  the  act  of  heat 
creation.1 

That  many  stars  are  moving  relatively  to  each 
other  is  a  matter  of  ready  demonstration  by  observa- 
tions of  their  positions  upon  the  sky,  with  the  instru- 
ments of  refinement  now  in  use,  at  intervals  of  a  few 
years.  Their  movement  may  be,  and  commonly  is, 
so  apparently  insignificant,  that  centuries  must 
elapse  before  their  displacement  would  be  apparent 
to  the  unaided  eye;  but,  upon  allowing  for  their 
excessive  remoteness,  speeds  are  revealed,  many 

i  To  the  term  motion,  a  somewhat  vague  one  as  used  generally,  science 
applies  a  definite  meaning,  the  product  of  mass  into  velocity.  The  function 
of  a  moving  body  that  is  in  direct  proportion  to  the  heat  developed  in  the 
alteration  of  its  speed  is,  however,  not  this  quantity,  but  the  product  of  its 
mass  into  the  square  of  its  velocity,  a  quantity  to  which  the  term  vis  viva 
was  formerly  applied.  One-half  of  the  vis  viva,  which  is  of  course  also 
proportional  to  the  heat  equivalent,  is  known  in  modern  mechanics  as 
kinetic  energy  and  is  of  great  importance. 


46  Recent  Advances  in  Astronomy. 

comparable  with,  and  some  far  greater  than,  those 
of  the  planets  in  their  orbits.  Sirius  drifts  over  the 
face  of  the  sky  with  such  speed  that  in  1400  years 
its  position  will  be  removed  from  its  present  one  by 
a  distance  that  would  just  be  covered  by  the  diameter 
of  the  Full  Moon.  From  the  known  distance  of  the 
star  it  is  a  simple  calculation  that  to  do  this  it  must 
travel  athwart  the  direction  of  vision  with  a  speed 
of  over  ten  miles  per  second,  more  than  one-half  of 
that  of  the  Earth  in  its  orbit;  and  this  takes  no 
account  of  any  velocity  the  star  may  possess  in  the 
direction  of  the  line  of  vision,  a  displacement  in 
which  direction  would  obviously  not  affect  its 
position  upon  the  face  of  the  heavens.  The  parallax 
of  Arcturus  is  inappreciable,  from  which  it  appears 
improbable  that  its  distance  can  be  less  than 
4,000,000  times  that  of  the  Sun;  thus  remote,  the 
drift  of  the  star,  by  which  it  would  be  carried  across 
the  diameter  of  the  Full  Moon  in  700  years, 
must  represent  a  velocity  of  at  least  130  miles  per 
second  across  the  line  of  sight;  the  actual  speed 
in  this  direction  being  greater  than  this,  in  direct 
proportion  as  the  actual  distance  of  the  star  exceeds 
the  minimum  limit  that  is  here  assigned  to  it. 
From  similar  considerations  it  appears,  that  in  the 
case  of  a  remarkable  star  in  the  Great  Bear  invisible 
to  the  naked  eye,  and  known  as  Groombridge  1830, 
from  the  number  assigned  to  it  in  Groombridge's 
catalogue,  the  speed  by  which  the  star  would  be 
carried  in  257  years  over  such  a  portion  of  the 
heavens  as  would  be  covered  by  the  Moon,  the 
most  rapid  displacement  known,  must  at  the  dis- 


The  Life  of  a  Star.  47 

tance  of  the  star  of  2,300,000  times  that  of  the 
Sun,  indicate  a  continual  rush  across  the  line  of 
sight  of  227  miles  per  second. 

The  velocity  of  the  Sun  relatively  to  the  stars,  or, 
more  definitely,  the  velocity  of  the  Sun  relatively  to 
the  mean  positions  of  the  stars,  a  quantity  com- 
monly alluded  to  as  "  the  velocity  of  the  Sun  in 
space ",  an  expression  almost  humorously  devoid 
of  meaning",  can  be  estimated  from  an  accumulation 
of  such  results  as  have  been  here  illustrated.  The 
problem  was  first  attacked  by  Sir  William  Herschel, 
and  has  ever  since  been  a  favourite  matter  of  re- 
search of  astronomers,  who  have  been  enabled  to 
introduce  increasing  refinements  as  more  and  more 
data  have  become  available.  All  the  methods  that 
have  been  applied  consist  essentially  of  the  deter- 
mination of  the  average  velocities  of  the  stars,  that 
is,  the  determination  of  the  velocity  of  the  average 
position  of  the  stars  relatively  to  the  Sun,  that  of 
the  Sun  relatively  to  the  mean  position  of  the  stars 
being  equal  and  opposite  to  this.  The  outcome  of 
such  investigations  seems  to  indicate  that  the  Sun 
is  travelling  in  a  line  directed  very  nearly  towards 
the  brilliant  star  Vega,  and  that  its  velocity  in  this 
direction  is  probably  between  12  and  18  miles  per 
second.  There  is  no  doubt  that  the  result  as 
regards  direction  is  far  more  definite  and  accurate 
than  that  as  regards  speed. 

In  the  host  of  the  " fixed  stars"  is  found  abun- 
dance of  motion,  and  that  upon  the  most  stupendous 
scale.  A  century  ago  it  was  fondly  hoped  that  the 
movements  of  the  stars  might  turn  out  to  be  of  the 


48  Recent  Advances  in  Astronomy. 

orderly  and  permanent  character  revealed  in  the 
Solar  System,  and  search  was  made  for  a  colossal 
Sun,  that  should  by  its  gravitational  attraction 
control  the  whole.  Sirius  was  suggested  by  Kant, 
other  stars  took  its  place  in  succession,  and  in  1846 
Madler,  abolishing  the  conception  of  a  central  Sun, 
imagined  that  every  member  of  the  stellar  host 
might  describe  an  orbit  about  a  centre,  placed  by 
him  in  the  Pleiades,  the  controlling  power  being, 
not  the  overpowering  attraction  of  one,  but  the  com- 
bined influence  of  all.  As  the  motions  of  the  stars 
became  more  closely  followed,  it  became  clear  that 
the  hope  of  revealed  order  was  not  destined  to  be 
realized.  System  remains  unrevealed  in  their  move- 
ments, and  the  stars  appear  to  rush  in  random 
directions  through  space. 

The  problem  before  us  then  is,  whether  in  their 
undirected  career  stars  may  not  from  time  to  time 
come  into  collision.  Were  the  Earth  in  its  orbital 
speed  to  meet  in  direct  impact  another  planet,  equal 
to  it  in  mass  and  travelling  with  an  equal  speed 
in  the  opposite  direction,  and  were  the  planets 
to  escape  being  shattered  into  fragments  by  the 
impact,  heat  would  be  developed  from  the  destruc- 
tion of  their  motion  sufficient  in  quantity  to  convert 
both  into  a  cloud  of  gas,1  and  it  is  conceivable  that 


i  The  collision  between  two  solid  planets  might  result  in  the  shattering  of 
considerable  portions  of  them  into  fragments,  and  in  the  fragments  being 
projected  into  space  with  high  velocities.  The  motion  retained  by  these 
fragments  would,  of  course,  escape  being  converted  into  heat.  In  the  case 
of  stars  that  had  not  cooled  so  far  as  to  reach  the  solid  condition,  such 
shattering  would  be  less  probable.  See  a  paper  by  Lord  Kelvin,  Popular 
Lectures  and  Addresses,  vol.  i.  p.  366. 


The  Life  of  a  Star.  49 

a  like  result  might  arise  from  collision  between 
stars.  From  the  insignificant  dimensions  of  the 
visible  stars  in  comparison  with  the  celestial  spaces 
in  which  they  have  their  being,  the  chance  against 
a  collision,  even  in  geological  ages,  is  perhaps 
excessively  remote;  but  in  indefinitely  prolonged 
time  collision  appears  certain.  It  must  be  remem- 
bered, in  addition,  that  the  stars  that  are  seen  may 
well  be  but  a  small  fraction  of  the  whole  system ; 
and  with  each  addition  of  dark  suns  the  probability 
of  collision  becomes  more  than  proportionately 
greater. 

Regarding,  however,  the  rejuvenescence  of  a  star 
by  collision  as  possible,  the  last  catastrophe  is  but 
projected  forward  by  a  finite  time.  At  each  collision 
the  coalescence  of  a  pair  of  cosmic  masses  will 
reduce  the  existing  number  by  one;  while  energy 
of  heat  is  gained  at  the  expense  of  energy  of  motion. 
As  aeon  succeeds  ason,  and  as  new  nebulas  follow 
those  from  the  ruins  of  which  they  were  formed  into 
extinction,  the  Universe  becomes  poorer  in  active 
energy;  and  there  appears,  so  far  as  physical 
science  has  interpreted  the  processes  of  nature,  no 
escape  from  the  picture  of  an  accumulation  of  inert 
matter  as  the  last  memorial  of  a  glorious  Universe 
of  Suns. 


(  M  520  ) 


50  Recent  Advances  in  Astronomy. 

Appendix  to  Chapter  I. 
The  Measurement  of  Stellar  Distances. 

Bessel's  discovery  of  stellar  parallax,  a  discovery 
that  directly  demonstrated  the  fact  of  the  Earth's 
annual  motion  round  the  Sun,  has  been  generally 
regarded  as  the  first  direct  proof  of  the  truth  of 
the  Copernican  System  of  Astronomy;  though  with- 
out doubt  a  very  strong  case  for  priority  in  this 
respect  might  be  made  out  for  the  detection  by 
Bradley,  rather  more  than  a  century  previously,  of 
the  aberration  of  light.  In  any  case,  however, 
Bessel's  achievement  removed  the  last  and  a  very 
serious  objection  to  the  Copernican  Hypothesis 
however  firmly  established,  and  has  rendered  it  in 
every  respect  unassailable.  The  discovery  itself 
must  take  high  rank  among  the  greatest  triumphs 
of  observation.  The  mere  detection  of  so  minute 
an  angle  as  even  the  relatively  large  parallax  of 
61  Cygni  still  necessitates  instrumental  means  of 
extreme  refinement,  as  well  as  very  great  observa- 
tional skill.  Two  lines  inclined  at  an  angle  of  a 
second  of  arc  would  approach  by  no  more  than 
i  inch  in  a  distance  of  3^  miles,  and  the  inclina- 
tion, not  only  detected  by  Bessel,  but  measured 
with  considerable  accuracy,  was  but  a  fraction 
of  this.  If  the  smallness  of  the  angles  con- 
cerned were  the  only  difficulty  in  observations  of 
stellar  parallax,  its  detection  would  be  no  mean 
feat;  but  the  observations  are  affected  by  numerous 


The  Measurement  of  Stellar  Distances.     51 

sources  of  error,  the  elimination  of  which  involves 
the  utmost  perseverance.  The  necessary  observa- 
tions must,  of  course,  be  made  at  widely  separated 
times  of  the  year,  and  difference  of  temperature  not 
unfrequently  causes  change  in  the  form  and  the 
position  of  the  observing  telescope,  that  would,  if 
not  taken  into  account,  completely  conceal  the 
insignificant  angle  of  parallax  by  simply  over- 
whelming it.  From  a  principle  similar  to  that  by 
which  the  rain-drops  of  a  falling  shower  appear  to 
slant  towards  a  moving  passenger  to  an  extent 
dependent  upon  the  rapidity  of  his  motion,  light 
rays  arriving  from  a  star  appear  to  an  observer 
upon  the  moving  Earth — as  he  is  carried  by  it  in 
its  orbital  rush  across  their  streams — to  slant  from 
the  direction  of  the  Earth's  motion,  and  the  star 
appears,  in  consequence,  to  be  displaced  toward 
that  point  in  the  heavens  to  which  the  Earth's 
motion  is  at  the  time  directed.  The  phenomenon 
is  known  as  "aberration  of  light";  it  was  indeed 
discovered  by  Bradley  in  1725  during  an  unsuc- 
cessful attempt  to  detect  the  parallax  of  a  star,  and 
the  displacement  in  the  apparent  position  of  a  star 
due  to  it  varies  from  nothing  to  20  seconds  of  arc 
according  to  the  direction  of  the  Earth's  motion 
with  respect  to  that  of  the  star.  In  estimating  the 
true  direction  of  a  star  from  its  apparent  place  in 
the  sky  it  is  obviously  necessary  to  take  the  most 
careful  account  of  the  aberration  of  light. 

The  apparent  position  of  a  star  is  also  seriously 
affected  by  refraction.  In  accordance  with  the 
general  fact  that  a  ray  of  light  is  deflected,  or 


52  Recent  Advances  in  Astronomy. 

refracted,  in  passing  from  a  medium  into  another 
differing  from  it  in  density,  the  refraction  being  to- 
wards the  perpendicular  to  the  separating  surface  as 
the  ray  passes  from  a  rarer  into  a  denser  medium, 
the  rays  from  a  star,  after  following  a  straight  course 
in  external  space,  are  deflected  downwards  on  enter- 
ing the  atmosphere,  and  as  the  air  continually  in- 
creases in  density  as  the  surface  of  the  Earth  is 
approached,  the  deflection  continually  increases,  so 
that  the  ray  reaches  an  observer  after  executing  a 
curve  in  the  atmosphere,  and  the  apparent  direction 
of  the  star,  determined  by  the  direction  of  the  ray 
on  entering  the  eye,  is  sensibly  different  from  its 
true  direction.  Unlike  the  interference  caused  by 
aberration,  which  may  be  corrected  from  an  exact 
knowledge  of  the  speed  of  the  Earth  relatively  to 
that  of  light,  the  error  due  to  refraction  is  incapable 
of  exact  determination,  since  the  curvature  of  the 
ray  is  dependent  upon  the  density,  temperature, 
and  degree  of  moisture  of  the  air,  not  only  in  the 
observatory,  but  throughout  the  whole  of  its  atmos- 
pheric path,  which  is  quite  beyond  the  reach  of 
observation. 

With  a  view  of  minimizing,  and  avoiding  as  far 
as  possible,  these  and  certain  other  sources  of  error, 
Bessel  adopted  in  his  observations  upon  61  Cygni 
a  method  originally  proposed  by  Galileo,  and 
known  as  that  of  "  relative  parallaxes".  In  it  no 
attempt  was  made  to  determine  the  absolute  direc- 
tion of  the  star  with  exactitude,  but  its  direction  was 
determined  with  a  very  high  degree  of  accuracy 
with  reference  to  a  neighbouring  "  reference  star", 


The  Measurement  of  Stellar  Distances.     53 

and  the  observations  were  repeated  at  different 
times  of  the  year.  The  important  assumption  was 
then  made,  and  its  justification  will  be  examined 
presently,  that  the  reference  star  was  so  extremely 
remote  that  the  direction  in  which  it  was  seen  was 
not  appreciably  af- 
fected by  the  Earth's  ,  i  L 

movement — in   other  ,'  /  /»' 

•  •  /  / 

words,    that    it    pos-  /  •/  ; 

sessed     no     parallax  ;  //  / 

that  could  be  detected  /  /  /  / 

— its  apparent  proxi-  /  /     /  / 

mity  to  the  star  under  /  / 

observation  being 
merely  the  result  of 
its  lying  by  chance 
nearly  in  the  same 
direction.  The  prin- 
ciple underlying  the 
application  of  the  ob-  p< 

SCrvationS  will  be  clear  Fig.  3.— Relative  Parallax. 

from  the  diagram  of 

fig.  3.  Let  the  two  parallel  lines  PC  and  QD  represent 
the  lines  of  sight  from  the  earth  in  its  two  positions  P 
and  Q  to  the  reference  star,  assumed  to  be  so  remote 
that  there  is  no  inclination  between  them  capable  of 
detection.  Then,  the  Earth  being  at  P,  let  the  direc- 
tion of  the  star  under  examination  be  observed  with 
reference  to  the  reference  star,  by  measuring  the 
angle  between  them.  Setting  off  this  angle  in  the 
figure  as  CPA,  the  direction  of  the  star  is  determined 
by  the  straight  line  PA.  Six  months  later,  the 


54  Recent  Advances  in  Astronomy. 

Earth  being  at  Q,  a  similar  observation  may  be 
made;  if  the  angle  separating  the  stars  be  observed, 
and  set  off  as  DQB,  the  direction  of  the  star  is  now 
indicated  by  the  straight  line  QB.  The  inclination 
of  PA  and  QB  determine  the  position  of  the  star  at 
their  meeting  point  at  s.  It  need  scarcely  be  re- 
marked that  in  actual  practice  the  position  and 
distance  of  the  star  are  determined  by  trigonometrical 
calculations  based  upon  the  observed  angles,  and 
not  from  graphical  construction  such  as  has  been 
here  introduced  to  illustrate  the  principle  involved. 
The  instrument  that  has  so  far  been  found  best 
adapted  to  the  measurement  of  the  required  angles 
is  known  as  the  heliometer,  from  the  fact  of  its 
having  been  originally  designed  to  determine  the 
angular  measure  of  the  Sun's  diameter ;  and  it  was 
with  such  an  instrument,  constructed  by  the  cele- 
brated optician  Fraunhofer  of  Munich,  that  Bessel's 
observations  were  made.  The  heliometer  is  a 
telescope,  the  object-glass  of  which  is  cut  into  two 
along  a  diameter.  With  the  two  halves  in  their 
normal  positions  each  may  be  regarded  as  giving 
a  separate  image  of  an  object  towards  which  the 
instrument  is  directed,  but  the  pair  of  images 
coincide,  and  under  these  conditions  the  heliometer 
is  equivalent  to  an  ordinary  telescope.  If,  however, 
one  of  the  halves  is  displaced  by  sliding  it  along 
the  line  dividing  the  pair,  the  image  formed  by 
it  is  equally  displaced,  and  the  observer  at  the 
common  eye-piece  sees  all  objects  in  the  field  of 
view  doubled,  as  if  viewed  through  a  crystal  of 
Iceland-spar.  In  practice  one  half  of  the  object- 


The  Measurement  of  Stellar  Distances.     55 

glass  is  displaced  until  the  image  of  the  star  under 
observation  for  parallax  formed  by  it  coincides  with 
the  image  of  the  reference  star  formed  by  the  other. 
The  amount  of  displacement  between  the  two  halves 
of  the  object-glass  then  indicates  the  angle  between 
the  directions  of  the  stars. 

The  advantage,  as  well  as  the  one  grave  dis- 
advantage, inherent  in  the  method  of  relative 
parallaxes  will  be  clear  upon  consideration.  The 
angles  CPA  and  DQB  are  not  appreciably  affected  by 
aberration,  since,  the  pair  of  stars  under  observation 
lying  in  nearly  the  same  direction  in  space,  their 
rays  will  traverse  closely  coincident  paths,  and  the 
apparent  change  in  their  direction  caused  by  the 
Earth's  rushing  across  the  streams  of  light  rays  will 
be  therefore  nearly  the  same  for  each.  For  a  similar 
reason,  error  due  to  refraction  is  practically  eli- 
minated, since  the  rays  from  the  two  stars  traverse 
nearly  the  same  column  of  atmosphere,  and  are 
therefore  refracted  by  it  almost  equally. 

It  is  important  to  notice  that  it  is  not  necessary 
to  know  the  absolute  directions  of  the  stars  with 
great  accuracy.  So  long  as  PC  and  QD,  the  lines  of 
sight  to  the  reference  star,  may  be  regarded  as 
parallel,  a  displacement  of  the  pair  of  them  to  and 
fro  even  through  several  degrees  makes  quite  an 
insignificant  change  in  the  estimated  distance  of  the 
star  at  s.  It  would  be  instructive  to  verify  this  by 
repeating  the  construction  for  directions  of  these 
lines  slightly  different  to  those  in  the  figure,  being 
careful  to  represent  them  as  parallel  in  every  case, 
and  to  retain  the  exact  values  of  the  angles  CPA 


56  Recent  Advances  in  Astronomy. 

and  DQB,  which  are  those  determined  by  the  helio- 
meter. 

The  one  serious  and  obvious  objection  to  the 
method  of  relative  parallaxes  lies  in  the  necessity 
for  assuming  that  the  reference  star  is  so  remote 
that  its  parallax  is  inappreciable.  It  will  be  readily 
seen,  by  modifying  the  construction  of  the  figure, 
that  if  the  lines  of  sight  to  the  reference  star  are 
inclined  to  each  other,  the  distance  of  the  star  s  will 
be  underestimated.  The  justification  for  the  method 
may  perhaps  be  stated  in  some  such  way  as  the 
following.  The  enormous  majority  of  the  stars 
show  no  parallax  relatively  to  each  other  that  can 
be  detected.  This  must  arise,  either  from  all  such 
stars  being  so  immensely  remote  that  their  paral- 
laxes are  represented  by  angles  so  small  as  to  be 
beyond  the  power  of  appreciation  even  by  the  helio- 
meter;  or,  if  the  stars  are  so  near  the  Earth  that 
these  angles  are  appreciable,  from  their  all  being 
equally  remote,  so  that  all  experience  equal  apparent 
displacements  when  regarded  from  different  points 
of  the  Earth's  orbit.  There  can  be  no  hesitation  in 
regarding  the  first  alternative  as  overwhelmingly 
probable.  There  is,  therefore,  a  great  probability 
that  any  given  star  selected  is  sufficiently  remote 
for  its  parallax  to  be  ignored  in  its  use  as  a  refer- 
ence star;  and  if  concordant  results  are  obtained 
from  the  employment  of  two  reference  stars,  which 
should  always  be  the  case  for  the  work  to  inspire 
confidence,  the  probability  of  the  soundness  of  the 
assumption  becomes  overwhelming. 

Before  the  application  by  Bessel  of  the  method  of 


The  Measurement  of  Stellar  Distances.     57 

relative  parallaxes,  the  attempt  had  generally  been 
made  to  determine  the  absolute  direction  of  the  star 
under  observation  for  parallax  by  recording  its 
position  upon  the  face  of  the  heavens  without  the 
assistance  of  reference  stars.  The  position  of  the 
star  was  determined  by  the  observation  of  its  "  right 
ascension"  and  " declination",  celestial  quantities 
quite  analogous  to  longitude  and  latitude  upon  the 
surface  of  the  Earth.  Such  a  method  is  known  as 
that  of  "  absolute  parallaxes",  and  it  is  preferable  to 
the  relative  method  in  that  it  does  not  involve  the 
aid  of  reference  stars.  Owing,  however,  to  observa- 
tions by  the  absolute  method  being  affected  to  the 
fullest  extent  by  refraction,  aberration,  and  other 
troubles,  as  well  as  from  its  involving  the  measure- 
ment of  large  angles,  which  are  far  more  difficult 
of  determination  within  the  same  limits  of  absolute 
error  than  small  ones,  the  possibility  of  its  success- 
ful application,  even  to  the  mere  detection  of  paral- 
lax, in  any  given  case,  is  so  extremely  remote  that 
it  has  now  been  universally  rejected.  It  is  true, 
Henderson's  discovery  of  the  large  parallax  of  a 
Centauri  was  effected  by  the  absolute  method,  but 
success  could  be  scarcely  anticipated  with  paral- 
laxes of  much  smaller  value. 

Bessel's  success  was  therefore  largely  due  to  the 
adoption  by  him  of  the  method  of  relative  parallaxes, 
as  well  as  to  the  fact  of  his  being  in  command  of  a 
fine  heliometer.  It  was  also  largely  due  to  the 
judicious  selection  of  a  star  for  examination.  Pre- 
vious to  Bessel's  measurements  there  was  reason  for 
regarding  is  as  probable  that  61  Cygni  was  one  of 


58  Recent  Advances  in  Astronomy. 

the  nearest  of  the  stars,  and  that  it  therefore  pos- 
sessed a  relatively  large  parallax.  In  appearance 
one  of  the  most  insignificant  and  unattractive  of  the 
stars,  attention  had  been  for  some  time  directed  to 
6 1  Cygni  by  reason  of  its  rapid  drift  across  the  face 
of  the  sky.  So  great  is  this  "  proper  motion"  in  its 
case  that  in  350  years  the  star  would  traverse  a  line 
in  the  heavens  equal  to  that  covered  by  the  diameter 
of  the  Full  Moon,  a  rapidity  of  movement  exceeded, 
so  far  as  is  known,  only  by  two  other  stars.  Since, 
other  conditions  remaining  the  same,  the  nearer  a 
moving  star  the  more  rapidly  would  it  appear  to 
drift  across  the  sky,  it  is  evidently  probable  that 
the  more  rapidly  drifting  stars  are  as  a  class  nearer 
than  the  others;  hence  in  the  search  after  the 
parallaxes  of  stars  special  attention  has  been 
directed  to  them,  and  61  Cygni  specially  attracted 
the  attention  of  Bessel.  Another  criterion  of  pro- 
bable nearness  is  supplied  by  brightness,  it  being 
a  priori  probable  that  the  brighter  stars  are  nearer 
the  Earth  than  fainter,  and  for  this  reason  special 
attention  has  also  been  devoted  to  them.  It  is  inter- 
esting to  notice  that  in  the  result  rapidity  of  motion 
has  proved  a  far  more  favourable  omen  of  success 
in  the  search  after  parallax  than  great  brilliancy. 


The  Milky  Way  and  Star  Distribution.      59 

Chapter  II. 
The  Milky  Way  and  the  Distribution  of  Stars. 

Among  the  many  and  profound  problems  sug- 
gested to  the  mind  by  the  contemplation  of  the 
heavens  upon  a  clear,  moonless  night,  there  is  no 
one  more  mysterious,  and  few  have  proved  more 
baffling,  than  that  presented  by  the  dimly-luminous 
arch  of  the  Milky  Way.  Variously  regarded  in 
classical  mythology  as  the  milk  that  flowed  from 
the  sacred  breast  of  Juno;  as  the  last  vestige  of 
the  ruin  that  overwhelmed  Phaeton  in  his  bold  but 
fatal  attempt  to  direct  the  fiery  steeds  of  the  Sun's 
chariot;  and  as  the  road  along  which  the  gods 
repaired  to  High  Olympus;  the  fair  shimmer  of 
the  Milky  Way  has  through  succeeding  ages  been 
associated  with  poetic  fancy  and  romantic  imagina- 
tion. In  modern  German  the  popular  term  "Jacob- 
strasse"  recalls  not  unfitly  the  sublime  vision  of  the 
patriarch  of  Israel;  while  to  the  Indian  of  North 
America  the  Milky  Way  is  still  the  path  of  departed 
souls,  and  the  brighter  stars  that  stud  its  stream  are 
the  camp-fires  that  mark  the  halting-places  of  his 
fathers  upon  their  weary  march. 

From  a  very  early  date  there  have  been  recorded 
speculations  of  a  more  or  less  scientific  nature  re- 
garding the  Milky  Way.  Pythagoras  is  recorded 
to  have  formed  the  shrewd  conjecture  that  its  faint 
shimmer  was  due  to  the  accumulated  light  of  multi- 
tudes of  faint  stars.  Anaxagoras  maintained  the 


60  Recent  Advances  in  Astronomy. 

view  that  the  appearance  might  be  caused  by  the 
projection  into  space  of  the  shadow  of  the  Earth  ; 
while  Aristotle  regarded  it  as  a  mist  formed  by  the 
exhalation  of  terrestrial  vapours.  That  it  was  a 
ring  of  nebulous  matter  in  external  space  encircling 
the  Earth  appeared  the  probably  correct  solution 
to  both  Tycho  Brahe  and  John  Kepler  toward  the 
close  of  the  sixteenth  century ;  but  a  few  years  later 
its  true  character  was  revealed  in  Galileo's  telescope, 
and  the  speculation  of  Pythagoras  was  confirmed  in 
the  discovery  that  its  haze  is  indeed  the  combined 
shimmer  of  hosts  of  stars,  each  one  too  faint  by 
itself  to  be  distinguished  by  the  unaided  eye. 

Seen  under  the  most  favourable  conditions  from 
these  latitudes,  the  Milky  Way  appears  as  a  semi- 
circular arch  of  light  spanning  the  starlit  sky. 
The  appearance  suggests  that  what  is  seen  is  the 
visible  half  of  a  complete  zone  encircling  the 
heavens,  the  other  half  being  at  the  time  of  obser- 
vation in  the  celestial  hemisphere  that  is  hidden 
by  the  solid  Earth  under  foot.  On  constructing  a 
complete  map  of  the  Celestial  Sphere,  and  tracing 
the  course  of  the  Milky  Way  upon  it,  it  is  seen  that 
this  view  is  roughly  correct;  but  the  entire  stream 
departs  from  a  simple  zone-like  character  in  several 
respects.  For  about  two-thirds  of  its  circuit  of  the 
heavens,  the  Milky  Way,  though  irregular,  appears 
as  an  unbroken  stream.  In  the  constellation  of  the 
Swan,  however,  it  bifurcates,  the  divided  branches, 
after  following  appreciably  parallel  tracts  for  about 
one  hundred  degrees,  reuniting  in  the  constellation 
of  the  Centaur.  The  division  in  the  Swan  is  excel- 


The  Milky  Way  and  Star  Distribution.      61 

lently  situated  for  observation  from  these  latitudes, 
from  which,  however,  the  reunion  in  the  Centaur 
is,  owing  to  its  proximity  to  the  South  Pole  of  the 
heavens,  permanently  invisible.  The  luminosity  of 
the  more  northerly  of  the  branches  of  the  divided 
stream  fades  at  a  short  distance  from  the  bifurcation 
in  the  Swan,  and,  indeed,  ceases  for  some  distance, 
while  it  is  very  remarkable  that  this  fading  is 
accompanied  by  an  increased  brilliance  in  the 
southern  branch.  That  the  branches,  though  separ- 
ated, are  not  physically  independent  is  indicated 
by  the  existence  of  imperfect  bridges  of  luminous 
matter  between  them,  while  in  several  points  the 
more  southern  and  brighter  branch  throws  off  com- 
paratively brilliant  projections  toward  its  companion, 
which  projections,  however,  terminate  before  reach- 
ing it. 

A  few  degrees  from  the  permanent  reunion  of  its 
divided  streams,  and  upon  entering  the  constellation 
of  the  Southern  Cross,  the  course  of  the  Milky  Way 
expands  into  a  brilliant  and  well-defined  cloud  of 
stars,  while,  in  the  centre  of  the  cloud,  closely  bor- 
dering upon  the  four  bright  stars  of  the  Cross,  is 
the  dark  pear-shaped  lake  popularly  known  as  the 
"  Coal  Sack  ".  The  "  Coal  Sack  "  is  one  of  a  great 
number  of  similar  irregularities  in  the  Milky  Way, 
though  in  no  other  is  the  passage  from  extreme 
richness  in  stars  to  almost  total  vacuity  so  sudden. 
The  appearance  of  a  dark  void,  unthinkable  in  ex- 
tent, in  the  midst  of  a  cloud  resplendent  with  the 
light  of  tens  of  thousands  of  suns,  is  indeed  one  of 
the  most  impressive  features  presented  by  the  system 


62  Recent  Advances  in  Astronomy. 

of  the  stars,  and  it  is  scarcely  remarkable  that  no 
explanation  of  it  has  been  advanced  that  it  is  not 
easy  to  refute  upon  the  most  elementary  grounds. 

A  third  and  no  less  remarkable  feature  of  the 
Milky  Way  is  presented  in  the  same  region  of  the 
heavens.  Following  the  course  of  the  Milky  Way 
past  the  Coal  Sack,  its  stream  contracts,  becoming 
almost  at  once  reduced  to  little  more  than  a  narrow 
neck  of  light.  Beyond  this,  however,  it  as  rapidly 
widens,  and  a  few  degrees  farther  on,  in  the  con- 
stellation of  Argo,  it  is  broken  clear  across  by  a 
dark  chasm.  In  Sir  John  Herschel's  beautiful 
drawing  of  the  Milky  Way  in  the  Southern  Hemi- 
sphere the  stream  upon  either  side  of  the  gap  is 
shown  as  extending  finger-like  projections  of  faint 
light  towards  the  opposite  side,  as  if  in  vain  en- 
deavours to  bridge  across  some  invisible  barrier, 
while  in  Gould's  more  recent  drawing,  executed  at 
Cordova,  multitudes  of  faint  stars  are  represented  as 
scattered  over  the  break. 

There  is  but  little  direct  evidence  as  to  the  distance 
of  the  Milky  Way.  It  is  conceivable  that  the  dis- 
tance of  a  portion  of  the  Milky  Way  might  be  re- 
vealed in  observations  for  parallax  made  upon  a 
star,  apparently  lying  in  its  stream  while  actually 
situated  far  beyond.  In  such  a  case,  if  a  number  of 
Milky- Way  stars  showed  the  same  relative  parallax 
with  reference  to  the  selected  star,  it  would  follow 
that  the  members  of  the  group  were  at  the  same 
distance  from  the  Earth,  and  if  the  parallax  of  the 
selected  star  were  inappreciable,  which  would  be 
probable  if  other  selected  stars,  similarly  examined 


The  Milky  Way  and  Star  Distribution.      63 

with  proper  precautions,  gave  the  same  result,  the 
distance  of  the  Milky- Way  stars  would  be  deter- 
mined. No  such  effect  as  this  has,  however,  been 
observed,  and  there  can  be  little  doubt  that  the 
Milky  Way  is  more  remote  than  at  least  many  of 
the  stars  whose  parallaxes  have  been  determined. 
There  is,  however,  no  known  reason  why  the  method 
should  not  be  successfully  applied  in  the  future. 

At  a  first  casual  glance  the  stream  of  the  Milky 
Way  appears  to  be  of  a  uniformly  diffused  lumi- 
nosity throughout.  Upon  more  careful  inspection, 
however,  the  first  suggestion  of  uniformity  dis- 
appears, and  a  vast  amount  of  varied  and  intricate 
structure  becomes  revealed  throughout  the  entire 
system.  Some  of  this  detail  is  readily  apparent  to 
the  naked  eye  upon  a  clear  moonless  night;  but,  to 
appreciate  it  in  its  full  beauty,  the  keenest  sight, 
supplemented  by  the  most  favourable  conditions,  is 
essential.  Such  conditions  involve,  of  course,  a 
moonless  night;  a  highly  transparent  atmosphere; 
a  time  when,  as  is  admirably  the  case  in  the  autumn 
and  winter  evenings,  the  Milky  Way  extends  high 
into  the  vault  of  heaven,  so  that  the  greater  part  of 
it  escapes  the  effects  of  light  absorption  by  the 
atmosphere,  always  so  appreciable  when  viewing 
celestial  objects  low  down  towards  the  horizon ;  and 
a  locality  well  removed  from  sources  of  artificial 
light,  and  the  consequent  glare  that  they  produce  in 
the  sky. 

As  the  eye,  under  such  conditions,  becomes  more 
and  more  accustomed  to  its  faint  light,  the  uniform 
luminosity  at  first  suggested  by  the  Milky  Way 


64  Recent  Advances  in  Astronomy. 

becomes  replaced  by  details  of  structure  steadily 
emerging  from  its  haze,  until  the  whole  stream 
spanning  the  sky  assumes  an  entirely  new  signifi- 
cation. Consisting  in  some  parts  of  clouds  of  faint 
stars,  separated  by  connected  dark  or  dusky  rifts; 
in  others,  of  wisps  of  starry  matter,  sometimes  inter- 
lacing in  inextricable  maze  over  the  body  of  the 
stream  itself,  and  frequently  projected  as  delicate 
streamers  far  into  the  neighbouring  sky;  the  whole 
appearance  of  the  Milky  Way  has  suggested,  not 
inaptly,  the  image  of  the  knotted  and  gnarled  trunk 
of  an  old  forest  tree.  The  interpretation  of  the 
scheme  thus  dimly  unfolded  may  be  beyond  our 
power,  but  in  surveying  it,  the  observer  becomes 
powerfully  impressed  with  the  conviction,  that  rather 
than  being  a  fortuitous  concourse  of  suns,  the  Milky 
Way  is  a  system  possessing  a  complicated  and 
varied  structure. 

The  telescopic  appearance  of  many  regions  of  the 
Milky  Way  is  of  extreme  beauty,  and  structure  is 
revealed  of  a  more  minute  character ;  that  appreci- 
able to  the  unaided  eye  being  not  unfrequently  lost 
owing  to  the  limited  area  of  the  sky  that  it  is  possible 
to  embrace  in  one  view.  Appearing  in  some  regions 
as  a  collection  of  individual  stars  scattered  apparently 
at  random  over  the  dark  background  of  the  sky ;  in 
others  as  clouds  of  innumerable  stars,  which  as  his 
telescope  moved,  suggested  to  Sir  John  Herschel 
the  image  of  a  drifting  scud ;  while  not  unfrequently 
its  hosts  of  stars  appear  as  if  involved  in  extensive 
nebulosity;  the  Milky  Way,  when  viewed  through 
a  fine  instrument  of  large  light-grasping  power, 


The  Milky  Way  and  Star  Distribution.      65 

presents  a  picture,  as  its  "  clusters  and  bee-like 
swarms  of  stars"  drift  in  silent  procession  across  the 
field  of  view,  that  is  in  the  highest  degree  impres- 
sive. To  the  true  star-gazer,  the  whole  picture 
possesses  an  inexpressible  and  quiet  charm,  and 
suggests  thoughts  to  which  few  would  find  it  easy 
to  give  expression. 

During  the  past  twenty  years  the  invention  of  the 
gelatine  dry  plate,  and  the  continual  improvements 
effected  in  its  preparation,  have  placed  a  new  method 
of  research,  and  one  of  tremendous  power,  at  the 
disposal  of  the  astronomer.  It  is  now  a  matter  of 
common  knowledge  that,  in  its  power  of  recording 
very  faint  light,  the  photographic  plate  far  surpasses 
eye  observation,  for  the  simple  reason  that,  to  be 
appreciated  by  the  eye  at  all,  an  object  must  be  of 
a  certain  brightness ;  but  that  the  effect  of  light  upon 
the  photographic  plate  being  cumulative,  a  percep- 
tible image  may  be  produced  by  light  far  below  the 
limit  of  visibility  to  the  eye,  if  its  action  upon  the 
plate  be  allowed  to  continue  for  a  sufficiently  long 
time.  It  is  consequently  in  the  representations  of 
extremely  faint  objects,  such  as  the  nebulas  and  the 
streams  of  the  Milky  Way,  that  photography  has 
achieved  its  greatest  triumphs  in  this  direction. 

In  its  application  to  astronomy,  the  photographic 
plate  is  usually  placed  within  a  telescope  from  which 
all  lenses,  with  the  exception  of  the  object-glass, 
have  been  removed;  and  at  a  distance  behind  the 
object-glass  equal  to  its  focal  length.  Under  these 
conditions  a  sharply-defined  image  of  the  celestial 
object  towards  which  the  telescope  is  directed  is 

(M520)  E 


66  Recent  Advances  in  Astronomy. 

formed  by  the  object-glass  upon  the  plate.  In  some 
of  the  most  beautiful  studies  of  the  Milky  Way, 
however,  a  portrait  lens  and  a  camera  closely  re- 
sembling the  usual  form  have  been  substituted  for 
the  telescope.  Larger  pictures  are  obtained  by  the 
former  method,  but  the  latter  embraces  a  larger  field 
of  view  and  is  the  more  sensitive.  It  is  scarcely 
necessary  to  add  that,  in  either  method,  the  entire 
instrument  must  be  mounted  appropriately  and 
driven  by  clockwork  so  as  to  accurately  follow  the 
celestial  object  in  its  apparent  diurnal  revolution 
round  the  Earth. 

Of  the  applications  of  the  photographic  plate  to 
the  study  of  the  minute  detail  of  the  Milky  Way  it 
will  only  be  necessary  to  refer  to  the  beautiful  work 
of  Mr.  Barnard  at  the  Lick  Observatory.  From 
the  glorious  situation  of  the  observatory  upon 
Mount  Hamilton,  under  conditions  nearly  perfect, 
so  far  as  atmospheric  transparency  is  concerned; 
with  11,000  feet,  and  that  the  most  troublesome 
portion,  of  the  atmosphere  below,  Mr.  Barnard, 
using  a  portrait  lens  of  only  6  inches  in  aper- 
ture, and  exposing  the  plates  for  periods  varying 
from  one  to  twelve  hours,  has  obtained  a  series  of 
pictures  of  the  Milky  Way  of  the  utmost  beauty 
and  delicacy.  It  is  scarcely  too  much  to  say  that 
the  pictures  as  far  surpass,  in  the  amount  of  detail 
revealed,  the  view  presented  to  the  eye  through  the 
largest  telescope,  as  does  the  latter  the  result  of 
naked-eye  observation;  though  it  must  be  con- 
fessed that  the  point-like  images  and  the  brilliancy 
of  the  star  pictures,  both  of  which  add  so  greatly 


The  Milky  Way  and  Star  Distribution.      67 

to  the  beauty  of  the  telescopic  view,  are  lost  in  the 
photograph. 

It  is  impossible  to  examine  the  exquisite  photo- 
graphs obtained  by  Mr.  Barnard,  even  superficially, 
without  being  impressed  with  the  sense  that  over 
enormous  regions  of  the  Milky  Way,  of  a  space  so 
vast  that  in  comparison  with  them  the  whole  of  the 
Solar  System  would  shrink  to  utter  insignificance, 
influences  have  been  at  work,  concerning  the  very 
nature  of  which  it  is  only  possible  to  form  the 
vaguest  conjecture.  The  most  striking  features  are 
perhaps  the  dark  lanes  or  rifts  that  so  frequently 
appear  to  intersect  clouds  of  stars.  These  rifts 
seldom  occur  in  isolation ;  they  more  generally  form 
branching  systems,  many  branches  frequently 
radiating  from  a  common  trunk  or  other  apparent 
vacuity.  In  some  cases  the  rifts  are  dark,  with 
sharply-defined  edges,  and  are  nearly,  or  quite, 
devoid  of  stars :  in  others,  they  are  uniformly  hazy, 
as  if  viewed  through  an  interposed  star  cloud: 
while,  again,  they  appear  of  dusky  and  less  regular 
forms  that  have  suggested  the  view  of  dark  clouds 
of  cosmic  dust  lying  between  the  Earth  and  a  more 
distant  background  of  brilliant  stars.  Another  very 
suggestive  feature  in  the  minute  structure  of  the 
Milky  Way  is  the  frequent  occurrence  in  it  of  stars 
arranged  in  lines.  The  lines  of  stars  are  frequently 
simple,  but  they  often  assume  curved  and  branch- 
ing forms,  and  a  very  general  characteristic  is 
shown  in  their  tendency  to  arrangement  in  direc- 
tions roughly  parallel  to  dark  rifts  in  the  same 
region.  A  remarkable  case  is  supplied  in  the 


68  Recent  Advances  in  Astronomy. 

constellation  of  Sagittarius,  where  Mr.  Barnard's 
photograph  shows  a  group  of  upwards  of  thirty 
small  stars  arranged  in  the  form  of  a  forked  twig, 
the  end  of  the  twig  remote  from  the  fork  being 
sharply  curved  round  into  a  hook.  In  the  imme- 
diate neighbourhood  of  this  group,  and  generally 
parallel  to  it,  are  several  well-marked,  dark,  and 
dusky  rifts.  In  many  cases  physical  relationship 
between  stars  thus  arranged  is  emphasized  by  the 
occurrence  of  streams  and  wreaths  of  nebulous 
matter  connecting  and  involving  the  stars.  Star 
streams  also  frequently  assume  the  form  of  closed 
oval  curves,  the  included  space  being  commonly 
darker  than  the  exterior. 

The  question  whether  the  Milky  Way  is  an 
isolated  structure  in  space,  or  whether  it  is  related 
in  any  recognizable  matter  to  the  system  of  the 
stars,  is  one  that  has  given  rise  to  much  study  and 
speculation  during  the  last  hundred  years.  To- 
wards the  close  of  the  last  century  Sir  William 
Herschel  noticed  that,  although  the  stars  were  dis- 
tributed over  the  face  of  the  sky  with  great  irregu- 
larity, there  was  upon  the  whole  a  decided  tendency 
to  increased  density  in  their  distribution  towards 
the  region  of  the  Milky  Way.  Confining  his 
attention,  by  reason  of  the  extended  nature  of  the 
problem,  to  a  zone  of  moderate  width  intersecting 
the  Milky  Way  at  right  angles,  and  directing  his 
reflecting  telescope  of  i87/io-mch  aperture  towards 
every  part,  in  succession,  of  more  than  one-half  of 
it,  upwards  of  three  thousand  observations  were 
made  of  the  number  of  stars  visible  at  any  one  time 


The  Milky  Way  and  Star  Distribution.      69 

in  the  field  of  view.  The  area  of  sky  embraced  in 
any  one  view  was  nearly  one-quarter  of  that  covered 
by  the  Full  Moon,  and,  taking  the  average  density 
of  star  distribution  for  equal  distances  from  the 
Milky  Way,  it  appeared  that  fewest  stars  appeared 
at  a  distance  of  90  degrees  from  it,  and  that  the 
number  continually  increased  as  its  stream  was 
approached.  In  equal  steps  of  15  degrees  each 
from  the  poles  of  the  Milky  Way  to  its  middle 
plane  the  average  numbers  of  stars  visible  in  the 
field  of  view  were  4,  5,  8,  14,  24,  and  53. 

In  recent  years  the  crowding  of  stars  towards  the 
zone  of  the  Milky  Way  has  been  examined  in  a 
more  detailed  manner.  Herschel  was  content  to 
merely  count  the  total  number  visible  at  any  one 
time  through  his  telescope,  ignoring  any  distinction 
as  regards  brightness  between  the  stars  themselves. 
In  1870  Mr.  Proctor  showed,  by  counting  the  num- 
ber of  lucid  stars,  i.e.  those  visible  to  the  naked 
eye,  that  a  similar  crowding  was  recognizable  with 
respect  to  them;  and  still  later  Mr.  Gore  has  shown 
that  a  similar  crowding  can  be  traced  in  stars  of 
each  individual  magnitude,  taken  separately.  Fur- 
ther, a  remarkable  law  of  crowding  becomes  ap- 
parent in  treating  the  problem  in  this  manner. 
With  the  brightest  of  the  stars  the  crowding  to  the 
Milky  Way  is  very  marked,  so  much  so  that,  as 
Mr.  Proctor  has  remarked,  upon  a  moonlit  night, 
when  the  Milky  Way  is  itself  invisible,  it  is  still 
possible  to  trace  its  course,  at  any  rate  in  the 
Southern  Hemisphere,  by  the  track  marked  by  the 
brilliant  stars.  With  descending  steps  in  brilliancy 


70  Recent  Advances  in  Astronomy. 

the  crowding  of  the  stars  becomes,  however,  less 
emphasized ;  for  stars  just  upon  the  limit  of  vision 
it  is  scarcely  recognizable,  while  for  telescopic  stars 
just  below  that  limit  it  practically  vanishes.  For 
still  fainter  stars,  however,  the  crowding  is  again 
apparent,  and  continues  to  become  more  and  more 
pronounced  until  the  faintest  of  the  telescopic  stars 
are  reached. 

It  is  easy  as  well  as  interesting  to  trace  this 
remarkable  law  of  distribution  in  the  positions  of 
the  brightest  stars.  Of  the  ten  most  brilliant  stars 
of  the  northern  hemisphere,  three — Capella,  Altair, 
and  Alpha  Cygni — are  situated  very  near  to  the 
central  line  of  the  Milky  Way,  though  its  entire 
stream  does  not  occupy  more  than  one-seventh  of 
the  hemisphere.  Four  others — Vega,  Procyon, 
Betelgeux,  and  Aldebaran — are  placed  upon  its  im- 
mediate border,  and  all  have  been  thought  to  be 
involved  in  its  faint  extensions.  A  zone  embracing 
four-fifths  of  the  sky,  with  the  Milky  Way  in  its 
middle  plane,  contains,  in  addition  to  these  seven, 
the  eighth,  Pollux,  that  is,  exactly  twice  as  many 
stars  as  it  should  if  the  distribution  had  been  uni- 
form. Two  only  of  the  ten,  Regulus  and  Arcturus, 
are  far  removed  from  the  Milky  Way,  and  it  is 
suggestive  that  of  this  pair,  Arcturus  is  notorious 
by  reason  of  its  very  high  proper  motion,  or  drift, 
across  the  face  of  the  sky ;  one  that  is  only  exceeded 
in  magnitude  by  eighteen  other  stars.  It  is  quite 
conceivable  that  an  enormous  speed  might  enable 
a  star  to  resist  a  tendency  to  distribution  to  which 
less  rapidly  moving  bodies  would  conform. 


The  Milky  Way  and  Star  Distribution.      71 

From  the  remarkable  law  of  star  distribution 
traced  by  Herschel  and  later  observers,  it  appears 
scarcely  possible  to  regard  the  Milky  Way  as  an 
independent  and  isolated  formation ;  while  a  definite 
relation  between  it  and  other  celestial  objects  is  still 
further  emphasized  by  a  study  of  the  distribution  of 
the  nebulae  in  space.  In  the  course  of  his  researches 
upon  these  celestial  clouds  Sir  William  Herschel 
became  aware  of  a  curious  antipathy  displayed  by 
them  towards  the  brighter  stars.  He  continually 
found  groups  of  nebulae  in  spaces  of  the  heavens 
comparatively  destitute  of  stars,  and  separated  from 
the  richer  regions  around  them  by  dark  spaces.  So 
firmly  did  he  become  impressed  with  the  reality  of 
this  relationship  of  avoidance,  that,  during  the 
process  of  "  sky  sweeping",  as  his  great  telescope 
was  directed  toward  different  regions  of  the  heavens, 
he  was  accustomed  to  warn  his  assistant  to  prepare 
to  record  nebulas,  for,  by  the  thinning  out  of  stars, 
he  anticipated  that  he  was  approaching  nebulous 
ground.  He  observed  that  the  crowded  stream  of 
the  Milky  Way  was  almost  destitute  of  nebulae, 
but  that  toward  its  poles,  where  stars  were  most 
sparsely  distributed,  nebulae  appeared  in  greatest 
number.  The  avoidance  of  the  Milky  Way  dis- 
played by  nebulae  was  still  further  emphasized  in 
the  observations  of  Sir  John  Herschel,  who  extended 
his  father's  researches  into  the  Southern  Hemi- 
sphere. 

The  avoidance  of  the  zone  of  the  Milky  Way 
displayed  by  nebulae,  and  their  condensation  toward 
its  poles,  may  be  very  strikingly  shown  by  con- 


72  Recent  Advances  in  Astronomy. 

structing  a  map  of  the  heavens,  upon  which  the 
positions  of  the  nebulas  and  that  of  the  stream  of 
the  Milky  Way  are  recorded.  Such  maps  have 
been  frequently  made,  the  most  recent  being  by  the 
late  Mr.  Sydney  Waters,  who  in  1893  marked,  upon 
the  method  known  as  that  of  "  equal  surface  pro- 
jection ",  the  positions  of  the  7840  nebulas  and  star 
clusters  recorded  in  Dr.  Dreyer's  New  General 
Catalogue  of  1888.  In  view  of  the  fact  that  at  a 
not  very  remote  date  the  belief  was  generally  enter- 
tained that  nebulas  were  clusters  of  stars,  it  is  in- 
teresting to  notice  the  relationship  displayed  by  star 
clusters  towards  the  Milky  Way.  Their  condensa- 
tion toward  it  is  even  more  pronounced  than  that  of 
the  stars,  very  few  being  found  beyond  the  limits 
of  the  Milky  Way,  while  for  a  considerable  part  of 
its  course  the  centre  of  its  stream  is  occupied  by  a 
continuous  line  of  them. 

The  application  of  the  spectroscope  to  the  study 
of  the  nebulas  has  brought  to  light  a  curious  modi- 
fication of  their  law  of  distribution.  We  have  seen 
in  the  previous  chapter  that  many  of  the  nebulas,  by 
yielding  a  broken  spectrum,  are  thereby  demon- 
strated, at  any  rate  so  far  as  their  luminous  con- 
stituents are  concerned,  to  be  in  a  gaseous  condition, 
but  that  the  light  from  others  yields  a  continuous 
spectrum,  which  is  at  present  incapable  of  exact 
interpretation.  On  examining  the  manner  in  which 
these  two  classes  of  nebulas  are  distributed  in  the 
heavens,  it  appears  that  the  " gaseous"  nebulas  share 
the  tendency  towards  condensation  displayed  by 
stars  in  crowding  towards  the  Milky  Way,  the 


The  Milky  Way  and  Star  Distribution.     73 

greater  number  of  nebulas  actually  appearing  in  it 
being  gaseous.  Upon  deducting  these  from  the 
total  number,  and  mapping  the  positions  of  the  re- 
mainder, the  avoidance  of  the  Milky  Way  displayed 
by  them  is  consequently  more  pronounced  than 
before. 

In  following  the  steps  by  which  structure  has  been 
traced  throughout  the  entire  stream  of  the  Milky 
Way,  and  system  revealed  in  the  distribution  of  the 
stars  and  nebulas  with  reference  to  it  and  to  each 
other,  we  have  so  far  followed  a  sure  course,  and 
indeed  have  had  occasion  to  do  little  more  than 
record  the  results  of  direct  observation.  That  stars 
and  nebulas  are  not  scattered  at  random  throughout 
space,  that  there  is  law  in  their  manner  of  distribu- 
tion upon  the  face  of  the  sky,  and  that  with  this  law 
the  stream  of  the  Milky  Way  is  in  some  manner 
intimately  associated,  cannot  be  doubted.  So  much 
is  known ;  and  it  is  scarcely  possible  to  refrain  from 
the  attempt  to  gain  some  closer  insight  into  the 
nature  and  meaning  of  the  entire  system.  Here, 
however,  our  steps  at  once  follow  a  less  certain 
track,  and  rapidly  lead  toward  a  region  of  specula- 
tion in  which  not  a  few  sound  thinkers  have  become 
sadly  bewildered. 

In  1784  Sir  William  Herschel,  adopting  in  prin- 
ciple a  suggestion  advanced  by  Thomas  Wright  of 
Durham  thirty-five  years  earlier,  attempted  to  ac- 
count for  the  appearance  of  the  Milky  Way,  and  the 
condensation  of  stars  toward  it,  as  a  perspective 
effect.  According  to  this  view,  all  stars,  including 
those  that  appear  to  form  the  Milky  Way,  were 


74  Recent  Advances  in  Astronomy. 

assumed  to  be,  upon  a  broad  average,  uniformly 
distributed  in  a  stratum  or  layer,  the  thickness  of 
which  was  small  in  comparison  with  its  dimen- 
sions in  its  own  plane;  while  the  Sun  was  situ- 
ated not  far  from  the  centre  of  the  stratum.  It 
is  clear  that  according  to  such  a  law  of  distribution 
the  line  of  sight  from  the  Solar  System  directed  at 
right  angles  to  the  stratum  would  soon  emerge  into 
external  space,  and  that  but  few  stars  would  appear 
in  this  direction ;  but  that  in  all  directions  parallel 
to  the  faces  of  the  stratum  stars  would  appear  to  be 
crowded,  since  the  line  of  vision  would  be,  in  all 
these  directions,  for  a  long  distance  involved  among 
stars.  All  round  the  Solar  System,  in  the  middle 
plane  of  the  stratum,  stars  would,  therefore,  appear 
to  be  crowded,  and  by  such  perspective  effect  the 
appearance  of  the  Milky  Way  was  imagined  to  be 
produced.  In  passing  from  a  direction  along  to 
one  at  right  angles  to  the  stratum,  the  length  of  the 
line  of  sight  included  in  it  would  continually  de- 
crease, and  fewer  and  fewer  stars  would  therefore  be 
seen  toward  the  poles  of  the  Milky  Way.  It  was 
further  suggested  that  the  bifurcation  of  the  Milky 
Way  was  an  optical  effect,  due  to  the  projection  from 
the  principal  stratum  of  a  secondary  one,  making 
a  small  angle  with  it,  and  leaving  it  nearly  in  the 
direction  of  a  straight  line  passing  from  the  Sun. 

If  the  stars  were  distributed  with  perfect  uniformity 
throughout  space,  and  if  the  most  distant  and  the 
faintest  were  visible  through  a  given  telescope,  it 
would  of  course  be  possible,  by  counting  the  num- 
bers visible  in  equal  areas  of  the  heavens,  to  compare 


The  Milky  Way  and  Star  Distribution.      75 

the  extensions  of  the  system  of  the  stars  in  the  cor- 
responding directions.  For  a  time  Herschel  believed 
that  these  conditions  were  fulfilled  with  sufficient 
approach  to  accuracy  to  justify  the  application  of 
the  method;  and,  with  a  view  of  " fathoming  the 
Universe  "  in  this  manner,  he  carried  out  a  laborious 
series  of  "  star-gages  ",  his  telescope  being  directed 
successively  to  upwards  of  3000  selected  regions  of 
the  heavens,  and  the  number  of  stars  in  each  field  of 
view  counted.  The  results  were  published  in  1785. 
It  follows  from  simple  geometry,  that  if  the  stars 
are  distributed  uniformly,  and  if  the  telescope  em- 
ployed is  sufficiently  powerful  to  reveal  all  of  them, 
the  extension  of  the  system  in  different  directions  is 
proportional  to  the  cube  root  of  the  number  of  stars 


Fig.  4. — Sir  William  Herschel's  earlier  view  regarding  the  Form  of 
the  Stellar  Universe. 

appearing  in  equal  areas  of  the  heavens  in  those 
directions;  and  upon  this  principle  Herschel  con- 
structed the  familiar  hypothetical  section  of  the 
sidereal  system  which  is  reproduced  in  fig.  4.  In 
this  the  Solar  System  occupies  the  position  indicated 
by  the  point  s,  the  straight  line  SA  is  directed  rather 
to  the  north  of  Sirius,  and  the  great  length  of  it  in- 
volved among  stars  gives  rise  to  an  optical  crowding 
to  which  the  appearance  of  the  Milky  Way  is  as- 
cribed; the  cleft  upon  the  opposite  side  accounts 


76  Recent  Advances  in  Astronomy. 

upon  the  same  principle  for  the  apparent  division  of 
the  Milky  Way  into  two  branches,  the  lines  SB  and 
sc,  by  fathoming  the  starry  extensions  shown  in  the 
section,  giving  rise  to  the  bifurcation  in  the  neigh- 
bourhood of  Altair. 

To  the  reader,  acquainted  with  the  intricate  struc- 
ture of  the  Milky  Way  revealed  by  more  recent 
observation,  it  will  be  at  once  apparent  how  com- 
pletely at  variance  with  nature  was  Herschel's  as- 
sumption that  its  appearance  could  be  due  to  any 
such  merely  optical  effect;  but  it  is  difficult  to 
imagine  how,  with  the  knowledge  of  star  distribution 
that  was  the  result  of  his  own  work,  Herschel  could 
have,  even  for  a  time,  regarded  his  fundamental 
assumption  of  approximately  uniform  distribution 
as  valid.  To  account  for  the  Coal  Sack  and  the 
other  less-pronounced  vacuities  of  the  Milky  Way 
upon  the  hypothesis  of  optical  effect,  vast  cone-like 
tunnels  must  be  imagined  as  converging  upon  the 
Solar  System  from  the  external  void;  while  every 
appearance  of  exceptional  richness  must  be  re- 
garded as  arising  from  immense  columns  of  stars 
projecting  from  the  system  afar  into  external  space, 
and  similarly  radiating  from  the  Sun.  As  a 
centre  of  such  converging  tunnels  of  vacuity  and 
radiating  projections  of  starry  cones,  the  Sun 
assumes  a  unique  place  in  the  Universe  entirely 
without  warrant.  Upon  the  optical  hypothesis, 
again,  except  upon  similarly  extravagant  assump- 
tions, the  appearance  of  crowding  of  stars  towards 
and  within  the  Milky  Way  should  take  place 
gradually.  From  the  external  sky  to  the  centre  of 


The  Milky  Way  and  Star  Distribution.      77 

its  stream,  density  of  star  distribution  should  in- 
crease continually  and  by  such  perfect  gradation 
that  it  should  be  impossible  to  define  the  limits  of 
the  Milky  Way.  Such  an  effect  is,  however,  directly 
contradicted  by  observation.  Throughout  its  entire 
course  the  boundary  of  the  Milky  Way  is  generally 
marked  with  fair  definition,  while  in  some  regions 
the  transition  from  its  star-crowded  depths  to  the 
external  sky  is  so  abrupt  that  one  half  of  the  field  of 
view  of  the  telescope  may  be  dark  while  the  other 
half  is  crowded  with  its  stars.  Such  features  point 
definitely  to  the  Milky  Way  being  a  real  and  definite 
structure,  separate  from,  though  doubtless  in  some 
way  closely  related  to,  external  celestial  bodies. 
That  this  conclusion  is  unavoidable  was  clearly 
recognized  in  his  later  life  by  Herschel  himself,  and 
was  fully  acknowledged  by  him,  although,  unfor- 
tunately, his  earlier  hypothesis  was  never  formally 
withdrawn. 

Closely  bearing  upon  the  problem  of  the  Milky 
Way  and  the  relation  of  external  objects  to  it,  is 
the  question  whether  the  stars  are  distributed  over 
a  finite  region,  or  whether  they  are  scattered  with- 
out limit  through  infinite  space.  In  other  words, 
is  the  visible  universe  finite  or  infinite  in  extent? 
Upon  the  random  suppositions  that  stars  are  distri- 
buted uniformly  and  without  limit  through  infinite 
space,  and  that  all  are  of  the  same  magnitude 
and  of  the  same  intrinsic  brilliancy  as  the  Sun, 
a  very  remarkable  conclusion  is  reached,  which  is 
often  quoted,  and  to  which  it  is  worth  while  to 
direct  attention.  With  the  Earth  as  centre,  let  an 


78  Recent  Advances  in  Astronomy. 

infinite  number  of  concentric  spherical  surfaces  be 
imagined  in  space  having  radii  proportional  to  the 
natural  numbers  i,  2,  3,  4,  &c.,  and  let  a  certain 
number  of  stars  be  imagined  to  be  distributed 
over  the  first  surface.  The  second  surface,  having 
twice  the  radius  of  the  first,  will  have  four  times 
the  area,  and  will  therefore  contain,  upon  the 
hypothesis  of  uniform  distribution,  four  times  as 
many  stars.  Since,  however,  the  apparent  disc 
of  each  star  would,  at  its  doubled  distance,  be 
diminished  in  area  to  one-fourth,  so  far  as  regards 
their  combined  areas,  the  increased  number  of  stars 
would  exactly  counterbalance  the  effect  of  their 
reduced  discs,  and  the  stars  upon  the  second  surface 
would  cover  the  same  extent  of  sky  as  those  upon 
the  first.  By  the  same  reasoning  this  would  also 
be  true  of  those  upon  the  third,  fourth,  and  every 
succeeding  surface;  so  that,  if  the  stars  extend 
through  space  without  limit,  upon  including  a 
sufficient  number  of  surfaces  the  whole  face  of  the 
sky  would  at  length  be  entirely  covered  by  stars. 
Further,  as  the  apparent  brightness  of  an  object 
does  not  diminish  with  its  distance,  unless  its  light 
undergoes  absorption  during  its  passage  from  the 
body  to  the  eye,  the  whole  vault  of  heaven  would 
be  both  by  day  and  by  night  resplendent  with  a 
dazzling  brilliancy  equal  to  that  of  the  Sun  itself — 
except  for  a  few  black  dots  marking  the  positions 
of  the  interposed  planets,  and  for  a  group  of  small 
dusky  objects,  spots  upon  the  Sun,  by  which  alone 
its  position  in  the  heavens  would  be  apparent. 
The  actual  appearance  of  the  heavens  is  so  strik- 


The  Milky  Way  and  Star  Distribution.      79 

ingly  different  from  this  that  it  is  clear  that  the  as- 
sumptions lying  at  the  base  of  the  deduction  are 
very  wide  of  the  mark. 

With  reference  to  this  reasoning  it  must  be 
noticed  that  the  apparent  brightness  of  a  surface  is 
independent  of  its  distance,  when,  and  only  when, 
there  is  no  absorption  of  light  in  its  passage  from 
the  surface  to  the  eye.  If  there  is  a  general  absorp- 
tion of  light  the  argument  fails.  Less  and  less 
light  would  be  received  from  increasingly  distant 
spheres,  and  the  combined  light  from  all  those 
beyond  a  certain  distance,  a  distance  depending  upon 
the  intensity  of  the  absorption,  would  be  a  negli- 
gible quantity.1  Absorption  of  light  in  interstellar 
space  might  result  from  imperfect  transparency  of 

1  The  demonstration  of  this  statement  is  comparatively  simple.  Suppose, 
as  a  concrete  case,  that  in  its  passage  from  the  nearest  of  the  spherical  sur- 
faces imagined  in  the  argument  to  the  eye,  %  of  the  light  that  would  if 
there  were  no  absorption  reach  the  eye,  is  actually  absorbed.  Then  if  L 
represents  the  whole  light  that  the  eye  would  receive  from  the  stars  upon  the 
first  surface  if  there  were  no  absorption,  the  light  actually  received  from 
them  would  be  %  L.  Consider  next  the  light  from  the  stars  upon  the  second 
surface.  Since  the  distance  separating  the  surfaces  is  equal  to  that  from 
the  first  surface  to  the  eye  ;  of  the  light  that  would  reach  the  first  surface 
from  the  second  on  its  way  to  the  eye  if  there  were  no  absorption  %  would 
escape  absorption,  while  in  its  farther  passage  to  the  eye  %  of  this  would 
escape,  the  light  arriving  being  therefore  %  of  %  L,  or  (?£)2x  L,  since  L 
is  the  same  as  before,  the  light  received  from  each  sphere  upon  the  assump- 
tion of  no  absorption  being  the  same.  By  similar  reasoning  the  light 
received  from  stars  on  the  third  surface  will  be  (^)s  L,  and  so  on,  the  total 
being 


an  infinite  series  of  terms,  the  value  of  which,  by  the  law  of  geometrical 
progression,  continually  approaches  without  limit,  but  can  never  exceed, 
2  L.  That  is,  for  this  special  value  of  absorption,  the  whole  light  from  an 
infinite  distribution  of  stars  would  be  only  double  the  amount  that  would  be 
received  from  the  stars  on  the  nearest  surface  if  there  were  no  absorption. 


8o  Recent  Advances  in  Astronomy. 

the  ether  of  space  by  which  all  radiation  is  trans- 
mitted, or  from  interposed  dark  matter  in  the  form 
of  dark  stars,  clouds  of  cosmic  dust,  or  swarms  of 
meteorites.  It  is  perhaps  too  much  to  regard  im- 
perfect transparency  of  the  ether  as  inconceivable ; 
but  should  absorption  in  the  ether  occur,  energy 
in  some  other  form  must  appear  equal  in  amount 
to  that  of  the  radiation  absorbed,  and  no  trace  of 
any  such  developed  energy  has  been  detected.  It 
has,  however,  been  shown  in  a  former  chapter  that 
the  existence  of  dark  matter  in  interstellar  space 
is  possible  if  not  probable,  both  in  the  form  of  dark 
stars  and  as  clouds  of  cosmic  dust  in  the  earliest 
stage  of  a  nebula,  while  meteor  swarms  are  an 
obvious  fact.  Light  absorption  might  arise  from 
all  or  any  one  of  these  causes,  cosmic  dust  clouds 
appearing  the  most  probably  effective,  so  that  the 
impossibility  of  an  infinite  distribution  of  stars  can- 
not be  demonstrated.  The  distribution  is,  however, 
clearly  not  uniform. 

In  spite  of  the  serious  objections  to  the  view  that 
the  density  of  star  distribution  upon  the  face  of  the 
heavens  can  be  taken  as  an  indication  of  the  exten- 
sion of  the  sidereal  system,  Sir  John  Herschel, 
during  his  residence  at  the  Cape  between  1834  and 
1838,  extended  his  father's  method  of  star-gaging, 
and  generally  maintained  the  view  that  the  appear- 
ance of  the  Milky  Way  is,  at  any  rate  in  part,  due 
to  optical  crowding.  The  comparatively  abrupt 
limits  of  its  stream  led  him,  however,  to  modify 
Sir  William  Herschel's  original  theory,  in  assum- 
ing the  stars  to  be  distributed  within  the  volume 


The  Milky  Way  and  Star  Distribution.      81 

of  a  flat  ring,  indefinitely  extended  in  all  directions, 
the  Solar  System  being  imagined  as  situated  near 
the  centre  of  the  hollow  of  the  ring.  By  splitting 
the  ring  in  its  medial  plane  nearly  to  its  centre, 
and  slightly  deflecting  outward  the  divided  portions, 
the  appearance  of  the  great  fissure  in  the  Milky 
Way  was  explained.  The  same  fundamental  con- 
ception underlies  the  modification  suggested  by 
Wilhelm  Struve  in  1847.  According  to  this  scheme 
the  whole  of  the  stars  were  distributed  in  parallel 
layers  or  strata,  the  strata  being  more  and  more 
densely  crowded  towards  a  central  plane  very  nearly 
passing  through  the  position  of  the  Sun.  Accord- 
ing to  this  view  there  was  a  real  condensation  of 
stars  towards  the  central  plane,  which  was  ap- 
parently increased  by  the  effect  of  optical  crowding, 
the  appearance  of  the  Milky  Way  being  the  com- 
bined effect  of  the  two  causes.  It  was  suggested, 
in  addition,  that  absorption  of  light  in  space  might 
reduce  the  last  effect  to  within  narrow  limits. 

To  these  modifications  of  Sir  William  Herschel's 
first  hypothesis  the  objections  urged  against  it, 
though  not  so  entirely  overwhelming,  are  neverthe- 
less fatal.  Both  are  equally  inconsistent  with  the 
appearance  of  the  Coal  Sack  and  other  vacuities, 
with  the  dark  rifts,  and  with  the  minute  detail 
revealed  by  later  researches.  According  to  each 
of  them,  the  transition  from  greater  to  less  density 
of  distribution  of  stars  beyond  the  limits  of  the 
Milky  Way  should  be  far  more  gradual  and  regular 
than  it  is.  Star-crowding  should  become  less  and 
less  pronounced  in  passing  from  the  limits  of  the 

(M620)  F 


82  Recent  Advances  in  Astronomy. 

Milky  Way  towards  its  poles;  but,  in  actual  obser- 
vation, while  this  is  found  to  be  the  case  so  far  as 
the  average  of  the  stars  is  concerned,  the  law  is 
very  far  from  being  maintained  in  isolated  regions. 
Several  regions  of  the  sky  in  close  proximity  to  the 
poles  of  the  Milky  Way  are  exceptionally  rich  in 
stars,  while  others,  closely  bordering  upon  its 
stream,  are  among  the  poorest  in  the  heavens. 
The  views  of  the  two  Herschels  and  Struve  are 
alike  untenable,  and,  since  the  publication  of  the 
last,  they  have  failed  to  find  support  save  in  the 
pages  of  certain  popular  works  on  astronomy. 

The  older  "disc"  and  "strata"  theories  having 
been  found  wanting,  the  Milky  Way  has  come  to 
be  generally  regarded  as  a  real  formation,  and 
attempts  have  been  made  to  construct  in  imagination 
a  stream  of  stars  that  should  give  rise  to  the  appear- 
ance actually  presented.  It  is  obvious  that,  in 
its  visible  aspect,  the  Milky  Way  appears  as  the 
projection  of  all  of  its  parts  upon  the  background 
of  the  sky.  Of  the  possible  depth  of  the  formation 
the  eye  takes  no  cognisance.  Its  many  apparently 
confused  and  interlacing  wreaths  may  be,  in  actual 
fact,  entirely  separate,  some  being  projected  inward 
toward  the  observer,  while  others  may  be  thrown 
backward  into  the  more  remote  spaces  behind. 

In  1869  Mr.  Proctor  attempted  to  show  that  the 
chief  features  of  the  Milky  Way,  and  more  particu- 
larly the  appearance  of  the  three  most  pronounced 
irregularities, — the  Coal  Sack,  the  bifurcation,  and 
the  great  Break  in  Argo, — might  arise  from  the  con- 
volutions of  a  single  stream  of  stars.  One  of  the 


The  Milky  Way  and  Star  Distribution.     83 

diagrams  accompanying  his  suggestion  is  repro- 
duced, with  a  slight  modification  in  the  arrangement 
of  the  lines  of  vision,  in  fig.  5.  In  this,  the  single 
stream  of  the  Milky  Way  is  represented  as  being 
somewhat  fantas-  C 

tically  curved,  the  ' 

ends  being  folded 
back  upon  the 
course  of  the  main 
stream.  The  Solar 
System  occupies 
the  position  indi- 
cated by  the  point 
s,  and  the  line  of 
sight  s  A,  escap- 
ing into  external 
space  between  the 
refolded  portions  of  the  stream,  accounts  for  the 
appearance  of  the  great  Break.  Close  to  the  Break, 
in  the  direction  SB,  the  two  portions  of  the  stream 
are  optically  superposed,  and  give  rise  to  the  bril- 
liant neck  in  Argo,  while  a  little  farther  on  one  or 
both  portions  diverge  from  the  median  plane  for  a 
short  distance,  producing  the  supposed  purely  opti- 
cal effect  of  the  Coal  Sack  in  the  direction  sc.  After 
a  farther  short  distance  through  which  the  two  por- 
tions continue  superposed,  and  through  which  the 
stream  consequently  appears  single,  apparent  divi- 
sion again  occurs  owing  to  a  second  divergence 
from  the  median  plane,  and  continues  until  the 
branches  reunite  along  SE  in  the  constellation  of  the 
Swan.  Of  these  apparent  branches,  one,  the  more 


Fig.  5. — Mr.  Proctor's  suggested  explanation 
of  the  Appearance  of  the  Milky  Way. 


84  Recent  Advances  in  Astronomy. 

northerly,  fades  and  becomes  for  a  time  invisible  by 
reason  of  excessive  distance,  while  the  other  in- 
creases greatly  in  brilliancy  owing  to  its  nearness. 
From  SE  to  SL  the  stream  appears  as  it  really  is, 
single;  but  beyond  SL,  and  extending  to  the  great 
Break,  the  series  of  lakes  or  vacuities,  which  are 
a  characteristic  feature  of  the  Milky  Way  in  this 
region,  are  accounted  for  by  repeated  temporary 
deviations  of  the  two  portions  from  the  median 
plane,  as  in  the  direction  SK. 

Mr.  Proctor's  view  of  the  formation  of  the  Milky 
Way  displays  the  ingenuity  that  would  have  been 
anticipated  from  so  sagacious  and  original  a  thinker, 
but  it  is  open  to  objections  of  so  serious  a  nature  as 
to  render  its  acceptance  impossible.  It  has  been 
urged  against  it,  that,  since  the  brightness  of  an 
object  does  not  diminish  with  increased  distance, 
the  fading  away  and  ultimate  disappearance  of  the 
northern  of  the  two  branches  bifurcating  in  the 
Swan  cannot  be  due  to  excessive  remoteness.  It 
has,  however,  been  seen  that  the  apparent  bright- 
ness of  an  object  is  only  independent  of  its  distance 
if  its  light  undergoes  no  absorption  in  space;  and 
since  such  absorption  has  been  shown  to  be  possible, 
if  not  probable,  a  fatal  attack  upon  Mr.  Proctor's 
scheme  cannot  be  maintained  upon  this  ground. 
It  is  curious  that  Mr.  Proctor  himself  makes  no 
reference  to  this  point,  though  he  was  beyond  all 
doubt  thoroughly  familiar  with  the  elementary  law 
in  question.  A  more  serious  objection  to  the  sug- 
gested explanation  lies  in  the  fact  that  it  is  scarcely 
possible  to  regard  the  divided  branches  of  the  Milky 


The  Milky  Way  and  Star  Distribution.      85 

Way  as  physically  independent.  For  a  consider- 
able distance  from  the  division  of  the  main  stream 
in  the  Swan,  the  southern  branch  continues  to  cast 
off  incipient  streamers  and  bright  projections  toward 
its  companion,  and  the  same  indication  of  intimate 
physical  connection  between  the  two  is  shown  in 
the  near  neighbourhood  of  the  reunion  of  the 
branches  in  the  Centaur.  It  might  seem  probable 
that  the  varying  apparent  breadth  of  the  Milky 
Way  might  serve  as  an  indication  of  the  proximity 
or  remoteness  of  different  parts  of  its  stream ;  but 
it  is  found  that  upon  the  whole  the  narrowest  por- 
tions, which  would  otherwise  appear  to  be  the  most 
remote,  are  the  brightest,  and  it  would  be  indeed 
difficult  to  regard  enhanced  brilliancy  as  generally 
associated  with  increased  distance. 

The  failure  of  these  and  other  attempts  to  explain 
the  appearance  of  the  Milky  Way,  and  of  the  irregu- 
larities displayed  by  it,  as  being  primarily  due  to 
perspective  effect  or  optical  projection,  point  to  the 
probability  of  the  more  simple  and  direct  view,  that 
the  Milky  Way  is  a  definite  and  complicated  struc- 
ture, and  that  its  bifurcation,  its  vacuities,  its  gaps, 
and  its  other  irregularities,  are  definite  physical 
facts.  To  this  view  astronomers  have  now  become 
reconciled.  Adopting  it,  the  sense  of  the  overwhelm- 
ing mystery  of  the  whole  undoubtedly  becomes 
greater,  while  it  must  be  confessed,  that  something 
is  gained  in  the  rejection  of  schemes  in  which  a 
rather  painful  suggestion  of  artificiality  somewhat 
conflicts  with  the  majesty  of  the  problem  toward  the 
partial  solution  of  which  they  have  been  directed. 


86  Recent  Advances  in  Astronomy. 

The  crowding  towards  the  zone  of  the  Milky  Way 
displayed  by  the  naked-eye  stars,  and  more  par- 
ticularly by  the  brightest  of  them,  suggests  that  their 
distribution  in  space  has  been  controlled  either  by 
the  Milky  Way  itself  or  by  the  influences  to  which 
it  owes  its  being.  Not  only,  however,  is  a  general 
relation  thus  indicated  between  the  external  stars 
and  the  Milky  Way,  but  it  has  been  maintained, 
notably  by  Mr.  Proctor  and  Mr.  Cowper  Ranyard, 
that  there  is  evidence  in  many  instances  of  a  more 
direct  and  intimate  relationship  between  separate 
stars  and  groups  of  stars  and  the  Milky  Way.  The 
evidence  consists  in  the  arrangement  of  bright  stars, 
both  individually  and  in  groups,  with  reference  to 
structures  in  the  Milky  Way.  While  acknowledging 
that  in  some  instances  it  is  difficult  to  avoid  a  feel- 
ing of  doubt  that  the  arrangements  in  question 
may  not  be  the  results  of  chance  distribution,  it  is 
scarcely  possible  not  to  recognize  in  others  very 
strong  evidence  of  intimate  physical  connection. 
It  must  be  sufficient  here  to  illustrate  the  general 
nature  of  the  arguments  adduced  by  reference  to  a 
few  selected  cases,  referring  the  reader  for  a  more 
complete  discussion  of  this  very  intricate  and  deli- 
cate subject  to  more  exhaustive  works.1 

The  constellation  of  the  Swan  lies  in  the  very 
heart  of  the  Milky  Way,  in  a  region  particularly 
interesting  from  the  evidence  of  structure  apparent 
in  it  both  to  the  naked  eye  and  in  photographic 

1  The  subject  is  developed  in  considerable  detail  in  the  chapter  on  ' '  The 
Stars  "  in  The  Old  and  New  Astronomy,  by  Proctor  and  Ranyard ;  and  has 
formed  the  subject  of  a  number  of  articles  that  have  appeared  in  Knowledge 
during  the  past  fifteen  years. 


The  Milky  Way  and  Star  Distribution.      87 

examination.  Upon  a  photographic  plate  exposed 
for  thirteen  hours  in  the  autumn  of  1891  by  Dr. 
Wolf  of  Heidelberg,  accumulations  of  stars  are 
shown  of  a  richness  unimaginable  in  the  finest 
telescopic  view,  while  throughout  vast  regions  the 
stars  appear  to  be  involved  in  a  faintly  luminous 
cloud.  This  enveloping  nebulosity  is  extensively 
furrowed  by  dark  lanes  and  rifts,  the  borders 
of  which  are,  in  the  manner  so  generally  charac- 
teristic of  them,  emphasized  by  lines  of  faint 
stars.  Conspicuous  in  Dr.  Wolfs  photograph  are 
the  images  of  the  bright  stars  «-  and  7  Cygni,  the 
two  that  form  the  head  and  centre  of  the  familiar 
cross  of  the  Swan.  Upon  the  photograph,  both  of 
these  stars  appear  to  be  nebulous,  their  blurred 
images  passing  by  insensible  gradation  into  the 
surrounding  cloud.  It  is  true  that  a  similar  appear- 
ance is  recognized  to  some  extent  in  all  photo- 
graphic pictures  of  bright  stars;  an  extension  of 
the  photographic  image  being  caused  by  the  dis- 
persion of  light  from  the  point-like  and  brilliantly 
illuminated  image  of  the  star  formed  upon  the 
sensitive  plate  during  exposure  into  the  sensitive 
silver  compound  around  it,  but  here  the  want  of 
definition  of  the  images  appears  to  be  far  more  than 
could  possibly  be  ascribed  to  this  cause,  and  the 
unsymmetrical  form  of  the  haze  by  which  7  Cygni 
is  enveloped  makes  it  scarcely  possible  to  accept 
such  explanation  of  the  appearance  in  its  case.  In 
addition,  the  position  of  a  Cygni  at  the  base  of  a 
remarkable  shrub-like  dark  formation  in  the  bright 
nebulosity,  and  that  of  7  Cygni  as  the  centre  of 


88  Recent  Advances  in  Astronomy. 

several  diverging  nebulous  structures,  point  strongly 
to  a  definite  physical  connection  between  both  stars 
and  the  apparently  surrounding  masses.  It  appears, 
therefore,  probable  that  the  stars  are  actually  bathed 
in  the  depths  of  the  Milky  Way  in  which  they 
appear,  and  do  not  owe  their  appearance  in  it  to  the 
chance  effect  of  optical  projection  upon  it. 

The  arrangement  of  many  of  the  stars  in  the  con- 
stellation Orion  is  very  remarkable.  The  proba- 
bility against  three  such  brilliant  stars  as  those 
forming  the  belt  falling  in  a  straight  line  and 
appearing  in  such  close  proximity  as  the  result  of 
chance  distribution  is  overwhelming.  The  close 
proximity  of  the  Great  Nebula  of  Orion,  as  well  as 
the  arrangement  of  many  of  its  contained  structures 
with  reference  to  the  direction  assumed  by  the  belt, 
is  moreover  suggestive ;  as  is  also  the  fact  that  the 
belt  is  situated  upon  the  immediate  border  of  the 
Milky  Way,  to  which  it  is  very  closely  parallel.  A 
group  of  five  faint  naked-eye  stars  lies  immediately 
below  the  belt;  they  are  arranged  in  a  line  parallel 
to  it,  and  it  is  easy  to  imagine  the  line  continued 
upwards  and  towards  the  west  by  a  farther  stream 
of  five  stars  separated  from  the  first  by  a  short 
break  and  deflected  northward  towards  the  Milky 
Way.  A  little  farther  to  the  west  a  second  stream 
of  naked-eye  stars,  at  least  thirteen  in  number, 
leaves  the  constellation  in  a  westward  direction, 
becoming  in  like  manner  deflected  to  the  north  and 
actually  penetrating  the  Milky  Way.  Both  of  these 
star  streams  are  represented  in  a  beautiful  drawing 
of  the  Milky  Way  as  seen  by  the  naked  eye,  re- 


The  Milky  Way  and  Star  Distribution.      89 

cently  executed  by  Dr.  Boeddicker,  as  involved  in 
its  cloudy  extensions.  To  the  last  point  it  may, 
however,  be  urged  with  considerable  force,  that  the 
appearance  of  a  faint  nebulous  stream  connecting 
the  members  of  a  line  of  faint  stars  may  be  an 
optical  illusion,  and  the  failure  of  the  photographic 
plate  to  verify  the  existence  of  the  cloudy  stream 
indicated  by  Dr.  Boeddicker  as  involving  the  line 
of  stars  immediately  below  the  belt,  shows  this  to 
have  been  the  case  in  this  instance.  From  the 
whole  of  the  evidence  it  will,  however,  probably  be 
conceded  that  the  configuration  of  the  stars  of  the 
belt,  and  the  symmetrical  arrangement  of  so  many 
conspicuous  stars  in  its  neighbourhood  in  lines 
having  the  same  general  trend,  demonstrates  the 
reality  of  a  close  physical  relation  between  the 
whole,  while  their  peculiar  relation  to  the  course 
of  the  Milky  Way  renders  probable  an  intimate 
relation  between  them  and  it. 

Mr.  Proctor  has  directed  special  attention  to  the 
close  association  of  lucid  stars  with  the  course  of 
the  Milky  Way  in  the  neighbourhood  of  the  Coal 
Sack.  Although  by  no  means  devoid  of  telescopic 
stars,  the  dark  area  of  the  Coal  Sack  does  not  in- 
clude one  visible  to  the  naked  eye.  If  the  naked- 
eye  stars  had  been  distributed  over  the  heavens 
with  uniformity,  the  number  assigned  to  the  Coal 
Sack  should  have  been,  at  the  least  estimate,  seven; 
and  their  avoidance  of  it  is  the  more  remarkable  in 
that  in  the  regions  immediately  surrounding  it  they 
are  particularly  richly  represented.  It  becomes, 
therefore,  scarcely  possible  not  to  regard  the  bright 


90  Recent  Advances  in  Astronomy. 

stars  as  intimately  associated  with  the  Milky  Way 
in  this  region,  and  as  being  situated  at  nearly  the 
same  distance.  The  brilliant  expansion  of  the 
Milky  Way  in  which  the  Coal  Sack  is  situated 
contains  the  five  bright  stars  of  the  Southern  Cross; 
and  of  these,  the  most  brilliant — the  first-magnitude 
star  a  Crucis — is  actually  upon  its  border.  From 
the  appearance  of  this  star  in  a  photograph  taken 
by  Mr.  H.  C.  Russell  at  Sydney  in  1890,*  Mr. 
Cowper  Ranyard  has  maintained  that  there  is 
strong  evidence  of  its  close  association  with  groups 
of  faint  stars  that  appear  upon  the  plate  in  its 
neighbourhood.  The  great  star  appears  to  be  the 
centre  of  several  diverging  streams  of  small  ones, 
while  other  groups  of  small  stars  are  arranged  con- 
centrically in  circles  round  it. 

The  apparent  luminosity  of  a  Crucis  cannot 
exceed  that  of  the  smaller  stars  by  less  than  three 
million  times,  so  that  if  it  be  regarded  as  probable 
that  all  are  equally  remote,  this  proportion  is  also 
that  of  their  actual  luminosities.  No  data  exist 
from  which  it  is  possible  to  form  an  estimate  of  the 
distance  either  of  a  Crucis  or  the  faint  stars,  from 
which  it  would  be  possible  to  compare  the  actual 
light-giving  powers  of  the  stars  with  that  of  the 
Sun ;  but  if  the  great  star  be  regarded  as  being  not 
very  different  from  the  Sun  in  light-giving  power, 
the  small  ones  need  scarcely  be  more  than  self- 
luminous  planets;  while,  if  the  small  ones  are  re- 
garded as  the  equivalents  of  the  Sun,  the  large  star 

1  The  photograph  is  produced  in  Knowledge  for  June,  1891,  and  in  The 
Old  and  New  Astronomy. 


The  Milky  Way  and  Star  Distribution.  ,   91 

must  be  of  such  luminosity  that,  for  its  surface 
brilliancy  to  be  no  more  than  equal  to  that  of  the 
Sun,  it  must  be  capable  of  enclosing  within  itself 
the  entire  orbit  of  Saturn.  Before  accepting  so 
astounding  a  view  as  the  last,  it  is  well  to  consider 
whether  adequate  grounds  exist  for  regarding  the 
former  as  improbable. 

From  the  accumulation  of  a  considerable  body  of 
evidence,  the  general  nature  of  which  has  been 
sketched  in  the  preceding  paragraphs,  an  intimate 
association  appears  probable  between  the  naked-eye 
stars  and  the  system  of  the  Milky  Way.  The  stars 
immediately  below  the  naked-eye  stars  in  brightness 
appear  to  be  influenced  by  its  course  to  a  far  less 
degree.  It  has  been  seen  that  the  tendency  to 
crowd  towards  the  zone  of  the  Milky  Way  practi- 
cally vanishes  with  stars  just  below  the  limit  of 
vision;  and  any  direct  relation  displayed  by  these 
faint  stars  towards  the  configuration  of  the  streams 
of  the  Milky  Way  is  difficult  of  detection.  Stars  of 
the  ninth  magnitude  are,  for  instance,  distributed 
with  apparent  uniformity  over  the  space  included 
between  the  divided  streams  between  the  Swan  and 
the  Centaur,  scarcely  if  at  all  less  richly  than  over 
the  branches  themselves;  and  faint  telescopic  stars 
are  scattered  with  approximate  evenness  over  the 
darkness  of  the  Coal  Sack.  Thus,  in  their  relation 
to  the  Milky  Way  the  naked-eye  stars  appear  to  be 
differentiated  from  those  immediately  below  them 
in  brightness.  Again,  since  the  apparent  brilliancy 
of  a  star  is  directly  affected  by  its  remoteness,  it 
appears  probable  that  the  brightest  among  the  stars 


92  Recent  Advances  in  Astronomy. 

are  upon  the  whole  those  that  are  nearest.  Since 
individual  stars,  as  has  been  seen,  vary  enormously 
in  magnitude,  exceptions  to  such  a  generalization 
would  of  course  be  expected;  they  are,  in  fact, 
directly  illustrated  in  the  near  proximity  of  so  in- 
conspicuous an  object  as  61  Cygni,  and  in  the  un- 
fathomable remoteness  of  Arcturus ;  but  it  would  be 
anticipated  that,  with  an  increasing  number  of  stars, 
such  irregularities  would  gradually  disappear  in 
a  general  average.  The  intimate  association  of 
the  brighter,  and,  therefore,  in  all  probability  the 
nearer,  stars  with  the  Milky  Way,  suggests  the 
view  that  the  Milky  Way  itself  is  a  comparatively 
near  neighbour  in  space. 

By  another  investigation,  proceeding  upon  en- 
tirely independent  lines,  we  are  also  led  to  regard 
it  as  probable  that  the  brighter  of  the  stars  are 
differentiated  from  the  rest  in  a  special  manner. 
From  the  time  of  Ptolemy  it  has  been  the  custom 
to  classify  stars  in  "  magnitudes"  in  the  order  of 
their  increasing  faintness.  To  very  bright  stars, 
such  as  Aldebaran  and  Altair,  a  position  in  the  first 
magnitude  is  assigned;  visibility  to  the  naked  eye 
terminates  under  favourable  conditions  at  about  the 
sixth  magnitude;  the  penetrating  power  of  the 
largest  telescopes  probably  reaches  stars  of  the 
fifteenth  magnitude,  while  the  power  of  the  photo- 
graphic plate  possibly  extends  to  some  four  or  five 
magnitudes  beyond.  Since,  however,  the  measure- 
ment of  the  brightness  of  a  star  is  a  matter  of  con- 
siderable delicacy,  and,  in  fact,  has  only  become 
satisfactorily  possible  in  recent  years,  magnitudes 


The  Milky  Way  and  Star  Distribution.     93 

assigned  to  stars  by  different  astronomers  have  been 
largely  a  matter  of  individual  judgment,  and  it  is 
not  surprising  that  the  scales  that  have  been  adopted 
are  very  conflicting.  Since  the  middle  of  the  pre- 
sent century,  however,  at  the  original  suggestion  of 
Pogson,  it  has  become  the  custom  to  apply  to  the 
term  magnitude  a  more  definite  meaning  than  it 
had  previously  received.  It  had  been  noticed  by 
Sir  John  Herschel,  that,  according  to  all  generally 
accepted  scales,  stars  of  the  first  exceeded  those  of 
the  sixth  magnitude  in  luminosity  very  nearly  100 
times,  and  Pogson  suggested  that  a  ratio  in  light- 
giving  power  of  100  to  i  should  be  regarded  as  the 
definition  of  a  difference  of  five  magnitudes,  the  four 
intermediate  magnitudes  being  interposed  in  such 
a  manner  that  stars  of  any  one  magnitude  should 
bear  a  constant  ratio  in  their  luminosity  to  those  of 
the  magnitude  following.  To  satisfy  this  condition, 
it  follows  that  any  star  must  exceed  in  brightness 
another  of  one  magnitude  fainter  by  2*512 .  .  times, 
this  "  light-ratio  "  being  the  fifth  root  of  loo.1  In 
the  result,  the  estimations  of  magnitude  by  the 
older  astronomers  are  found  to  conform  very  closely 
to  the  absolute  scale  so  far  as  the  naked-eye  stars 
are  concerned,  but  to  deviate  considerably  from  it 
for  fainter  ones. 

From  the  absolute  definition  of  star  magnitude  it 
is  possible  to  calculate  the  relative  numbers  in  which 

1Thus  a  first-magnitude  star  is  equivalent  to  2  '5 12  second-magnitude 
stars,  a  second-magnitude  star  to  2-512  third-magnitude  stars,  and  so  on. 
Hence  a  first-magnitude  star  is  equivalent  to  2*512  x  2'5i2  or  (2'5i2)2  third- 
magnitude  stars,  (2'5i2)3  fourth-magnitude  stars,  and  (2'5i2)°  or  100  sixth- 
magnitude  stars. 


94  Recent  Advances  in  Astronomy. 

stars  of  different  magnitudes  should  appear  in  the 
heavens  upon  the  assumption  of  their  uniform 
distribution  in  space.  From  the  result  of  the 
calculation,  the  details  of  which  may  be  left  as  an 
exercise  in  geometry  to  the  reader  and  do  not 
present  any  serious  difficulty,  it  appears  that  if  a 
number  of  stars,  either  of  the  same  or  different 
degrees  of  intrinsic  brilliancy,  were  scattered  in 
space,  subject  only  to  the  condition  that  those  of 
each  degree  of  luminosity  were  distributed  uniformly, 
there  should  appear  nearly  four — more  accurately 
3*981 — times  as  many  stars  of  any  given  magni- 
tude as  of  the  next  exceeding  it  in  brightness.  For 
every  star  of  the  first  magnitude  there  should 
appear,  for  instance,  nearly  four  stars  of  the  second, 
sixteen  of  the  third,  and  sixty-four  of  the  fourth. 
It  cannot  fail  to  be  interesting  to  compare  these 
numbers  with  those  actually  observed. 

Data  for  such  a  comparison  are  supplied  in  star 
catalogues.  Of  these  the  most  extensive  that  has  so 
far  been  constructed  is  known  as  the  Bonn  Durch- 
musterung.  It  was  compiled  under  the  supervision 
of  Argelander  between  1859  and  1862,  and  in  it  are 
recorded  the  positions  and  magnitudes  of  324,198 
stars, — all  those  of  the  northern  hemisphere  down 
to  the  9*5  magnitude.  The  more  recent  catalogue 
constructed  at  Harvard  by  Professor  E.  C.  Pickering, 
giving  the  magnitudes  of  stars  as  measured  by  the 
meridian  photometer,  is  undoubtedly  the  more 
accurate  in  this  respect,  but,  since  it  includes  only 
those  brighter  than  the  6*5  magnitude,  it  is  not  so 
well  adapted  to  the  present  purpose.  The  same 


The  Milky  Way  and  Star  Distribution.     95 

general  result,  however,  appears  from  the  adoption 
of  the  data  of  either  the  Bonn  or  the  Harvard  cata- 
logues, as  well  as  from  Dr.  Gould's  catalogue  of 
stars  visible  from  the  southern  hemisphere. 

The  result  of  the  comparison  is  expressed  in  the 
following  table.  In  the  second  column  are  given 
the  numbers  of  stars  between  the  limits  of  magni- 
tude indicated  in  the  first.  These  numbers  are  given 
by  Littrow  as  the  result  of  his  examination  of  the 
Bonn  Durchmusterung.  The  third  column  contains 
the  numbers  of  stars  that  should  have  appeared 
upon  the  hypothesis  of  uniform  distribution ;  and  the 
last,  numbers  obtained  by  dividing  the  figures  in 
the  second  column  by  those  in  the  third,  that  is,  the 
numerical  proportion  of  stars  actually  observed  to 
those  that  should  have  been  observed  upon  the 
uniform -distribution  hypothesis,  a  quantity  that 
may  be  conveniently  described  as  the  "  apparent 
crowding  ". 

COMPARISON  BETWEEN  THE  OBSERVED  NUMBERS  OF  STARS  OF 
DIFFERENT  MAGNITUDES  WITH  THE  THEORETICAL  NUMBERS 
UPON  THE  HYPOTHESIS  OF  UNIFORM  DISTRIBUTION  IN  SPACE. 


Limiting 
magnitudes. 

Numbers  of 
stars  actually 
observed. 

Theoretical 
numbers. 

Apparent 
crowding. 

I  to  2 

IO 

4 

2  '5 

2  to  3 

37 

15 

2-46 

3  to  4 

130 

58 

2-24 

4  to  5 

312 

234 

i'33 

5  to  6 

I,OOI 

931 

i  -08 

6  to  7 

4,386 

3,705 

1-18 

7  to  8 

13,822 

M,75I 

'94 

19,698 

19,698 

96  Recent  Advances  in  Astronomy. 

The  general  result  of  the  comparison,  as  ex- 
pressed in  the  figures  of  the  last  column,  is  very 
remarkable.  There  appear  to  be  many  more  stars 
of  the  first  five  magnitudes  than  there  should  be 
according  to  the  hypothesis  of  their  uniform  distri- 
bution in  space.  Rejecting  the  stars  tabulated 
between  the  limits  of  the  first  and  second  magni- 
tudes as  being  too  few  from  which  to  draw  any 
reliable  deduction,  and  also  as  comprising  several 
stars,  such  as  Arcturus  and  Vega,  which  should  in 
strictness  be  excluded,  as  being  too  bright  to  be 
regarded  even  as  first-magnitude  stars,  there  is 
apparent  in  the  record  a  crowding  of  the  brighter 
stars,  that  diminishes  with  increasing  faintness  and 
becomes  insignificant  beyond  the  limit  of  the  fifth 
magnitude — that  is,  near  the  limit  of  visibility  to 
the  unaided  eye.  It  has  been  stated  that  a  similar 
appearance  of  crowding  is  recognizable  in  the  more 
exact  record  of  star  magnitudes  contained  in  the 
Harvard  catalogue,  as  well  as  in  Dr.  Gould's 
catalogue  of  stars  of  the  southern  hemisphere. 

An  apparent  crowding  of  the  brighter  stars 
would  be  quite  consistent  with  their  uniform  dis- 
tribution if  light  experienced  absorption  in  space, 
since  such  absorption  would  affect  the  light  from 
distant  stars  to  a  greater  extent  than  that  from 
nearer  ones.  This  explanation,  however,  scarcely 
appears  to  be  applicable  here,  since  from  the  fifth 
magnitude  to  the  eighth,  stars  appear  in  numbers 
not  very  different  from  those  estimated  upon  the 
assumption  of  their  uniform  distribution.  Were 
there  appreciable  light  absorption  within  the  limits 


The  Milky  Way  and  Star  Distribution.      97 

of  space  in  which  stars  as  far  as  those  of  the  eighth 
magnitude  are  distributed,  its  effects  should  be  as 
conspicuous  in  the  falling  off  of  numbers  in  succes- 
sive magnitudes  among  the  fainter  as  in  brighter 
magnitudes. 

The  most  simple  view  to  take  with  reference  to 
the  apparent  crowding  of  the  brighter  stars  is  that 
it  results  from  a  real  crowding  of  stars  in  the  neigh- 
bourhood of  the  Sun.  There  is  nothing  inherently 
improbable  in  this  view,  since  the  study  of  the  sky 
reveals  numerous  analogous  instances  of  the  cluster- 
ing of  stars.  Setting  aside  such  extreme  cases  as 
are  presented  in  the  Pleiades,  the  Hyades,  and 
other  such  strongly  pronounced  clusters,  many  rich 
regions  of  the  heavens,  not  even  included  in  the 
Milky  Way,  furnish  instances  in  which  the  local 
density  of  star  distribution  is  far  in  excess  of  that 
which  could  have  resulted  from  chance  distribution. 

The  suggestion  of  a  clustering  of  stars  in  the 
neighbourhood  of  the  Sun  acquires  additional 
interest  from  the  indications  already  recognized, 
that  the  nearer  among  the  stars  are  differentiated 
from  the  rest  by  an  intimate  association  displayed 
by  them  towards  the  stream  of  the  Milky  Way.  It 
appears  scarcely  possible  not  to  recognize  in  the 
complete  testimony  a  suggestion  that  the  Sun  is  a 
member  of  a  star-cluster,  one  in  which  the  Milky 
Way  is  involved  as  a  stream  of  stars  far  smaller  than 
the  more  conspicuous  members  of  the  cluster,  but 
closely  associated  with  the  fundamental  scheme  of 
its  structure.  According  to  such  view  the  appear- 
ance of  uniformity  in  the  distribution  of  the  stars 

(M520)  G 


98  Recent  Advances  in  Astronomy. 

from  the  fifth  to  the  eighth  magnitude,  as  well  as 
their  comparative  indifference  to  the  zone  of  the 
Milky  Way,  are  alike  due  to  their  lying  for  the 
most  part  beyond  the  region  in  which  the  clustering 
tendency  and  the  apparently  attractive  influence  of 
the  Milky  Way  extends.  The  Milky  Way,  to- 
gether with  the  cluster  containing  the  Sun,  may 
conceivably  constitute  a  true  independent  system, 
while  it  is  possible  that  similarly  associated  with 
other  star-clusters  there  may  exist  other  streams  of 
star-dust,  undistinguishable  from  their  excessive 
remoteness. 

Before  regarding  this  speculation  as  probable,  it 
is  essential  to  imagine  some  possible  explanation 
of  the  crowding  towards  the  Milky  Way  again 
exhibited  by  the  still  fainter  telescopic  stars.  It  is 
not  inconceivable  that  this  appearance  may  be  due 
to  the  escape  of  true  Milky- Way  stars  from  within 
its  stream  into  external  space.  It  is  not  possible 
here  to  develop  this  suggestion  fully,  but  it  would 
appear  probable,  that,  if  a  number  of  stars  were 
distributed  at  random  and  with  random  velocities 
both  as  regards  magnitude  and  direction  through 
a  definite  region  in  space,  a  condition  of  things 
would  result  not  unlike  that  imagined  in  the  kinetic 
theory  of  gases.  Pairs  of  members  of  the  swarm 
would  from  time  to  time  approach  so  closely  as  to 
describe,  under  the  influence  of  their  mutual  gravi- 
tation, hyperbolic  orbits  round  each  other,  the 
common  centre  of  mass  of  the  pair  marking  the 
position  of  a  common  focus.  If,  by  chance,  it 
happened  that  the  masses  and  velocities  of  the  pair 


The  Milky  Way  and  Star  Distribution.     99 

were  so  related  that  their  centre  of  mass  was  at  rest, 
the  direction  only  of  the  star  motions  would  be 
affected  by  their  near  approach,  the  velocity  of  each 
being  reduced  upon  separation  to  an  extent  equal 
to  its  increase  upon  approach;  but  if,  as  would 
generally  be  the  case,  the  centre  of  mass  was  in 
motion,  since  the  velocities  of  the  stars  would  be 
ultimately  unaffected  with  reference  to  it,  the  actual 
velocities  would  be  changed,  the  star  receding 
after  the  "  encounter"  in  the  direction  of  motion  of 
the  centre  of  mass  having  its  velocity  increased, 
while  the  speed  of  the  other  would  have  become 
less.  Encounters  between  stars  continually  occur- 
ring, all  velocities,  without  limit  of  magnitude, 
would  be  continually  being  produced  in  the  cluster, 
and  from  time  to  time  a  star  would  acquire  sufficient 
speed  to  carry  it  beyond  the  limits  of  the  cluster, 
while  its  speed  might  be  so  great  as  to  place  it 
beyond  the  controlling  influence  of  gravitation,  in 
which  case  it  would  leave  the  cluster  never  to 
return.  The  system  would  slowly  disintegrate,  and 
during  the  process  the  escaped  members  would  be 
found  scattered  in  external  space  most  densely 
distributed  in  the  immediate  neighbourhood  of  the 
original  swarm. 

Similar  encounters  must  occasionally  take  place 
between  members  of  the  main  cluster  in  which  the 
Milky  Way  is  involved,  which,  by  similar  reasoning, 
it  is  scarcely  possible  to  regard  as  a  stable  system. 
The  extreme  velocity  of  such  "  runaway"  stars  as 
1830  Groombridge  may  well  be  due  to  their  having 
experienced  a  number  of  favourable  encounters  with 


ioo          Recent  Advances  in  Astronomy. 

other  stars,  and  in  any  case  does  not  point  to  their 
being,  as  has  been  suggested,  temporary  visitors  to 
the  system  of  the  visible  stars  from  external  regions 
of  space,  ploughing  their  way  through  it  by  reason 
of  enormous  initial  speeds  incapable  of  generation 
by  the  gravitational  attraction  of  the  system.  It  is 
conceivable  that  in  the  remote  past  the  sun-cluster 
may  have  been  far  richer  than  it  is  now,  and  the 
firmament  may  have  been  more  resplendent  with 
brilliant  stars,  but  that  from  age  to  age  its  members 
may  have  been  gradually  scattered,  and  the  vault  of 
heaven  may  now  be  growing  poorer. 

It  is  scarcely  necessary  to  remind  the  reader  that 
in  this  chapter  no  attempt  has  been  made  to  explain 
the  function  of  the  Milky  Way,  or  its  connection 
with  the  stars.  An  attempt  only  has  been  made  in 
the  latter  pages  to  define  its  possible  relation  to  the 
stars,  and  it  is  not  suggested  that  the  attempt  ex- 
tends beyond  the  limits  of  speculation.  Ignorance 
of  the  distances  of  more  than  an  insignificant 
minority  of  its  members  appears  at  present  an  in- 
superable obstacle  towards  extending  the  web  of 
exact  knowledge  far  into  the  system  of  the  stars; 
but,  in  the  absence  of  more  exact  methods,  it  is 
impossible  for  the  lover  of  the  picture  of  infinite 
grandeur  and  majesty,  mapped  out  night  by  night 
upon  the  fair  face  of  the  starlit  sky,  to  refrain  from 
indulging  in  some  conjecture,  however  vague  and 
in  itself  unsatisfactory,  as  to  the  meaning  of  so  ex- 
quisitely beautiful  and  mysterious  a  record. 


The  Recent  Study  of  Mars.  101 

Chapter  III. 
The  Recent  Study  of  Mars. 

From  the  time  that  the  telescope  revealed  to  Sir 
William  Herschel  the  first  clear  picture  of  the  planet 
Mars,  and  led  him  to  regard  the  details  of  the 
delicately-tinted  image  presented  to  his  view  as 
indicating  the  existence  upon  the  surface  of  the 
planet  of  physical  conditions  not  very  unlike  those 
familiar  to  the  inhabitants  of  the  Earth,  a  special 
interest  has  attached  to  this,  the  only  one  of  the 
orbs  of  heaven  that  it  is  possible  to  contemplate 
with  any  degree  of  confidence  as  a  sister  world.  In 
their  distribution,  general  configuration,  and  colour, 
the  planetary  markings  have  appeared  during  the 
greater  part  of  the  present  century  to  harmonize 
well  with  their  tempting  interpretation  as  oceans, 
continents,  and  polar  regions  bound  in  eternal 
snow.  The  rotation  of  the  planet  and  the  haze  in 
which  many  of  its  features  appeared  to  be  en- 
veloped indicate  the  regular  succession  of  day  and 
night,  each  passing  into  the  other  by  the  insensible 
gradations  of  morning  and  evening  twilight;  while 
the  tilt  of  the  axis  of  rotation  of  the  planet  to  the 
plane  of  its  orbit  demonstrates  the  constant  recur- 
rence of  seasons.  In  recognizing  upon  a  planet 
so  many  conditions  essential  to  its  well-being  as  a 
world,  it  has  been  impossible  to  restrain  imagina- 
tion from  supplementing  actual  discovery  in  regions 
lying  beyond  the  power  of  telescopic  observation. 


102          Recent  Advances  in  Astronomy. 

The  lands  and  waters  of  Mars  teemed  with  animal 
and  vegetable  life.  In  lands  over  which  the  cold 
and  heat  of  winter  and  summer  and  day  and  night 
never  ceased,  seed-time  and  harvest  were  added; 
while,  passing  their  brief  span  of  struggle  and 
passion,  and  fighting  to  maintain  their  mastery  over 
nature,  were  intelligent  beings,  towards  whom,  in 
imagination,  the  right  hand  of  fellowship  was 
longingly  extended  across  a  separating  chasm  of 
nearly  50,000,000  miles. 

For  the  greater  part  of  the  century  that  followed 
Herschel's  observations,  although  knowledge  of 
Martian  detail  steadily  increased,  little  was  added 
to  it  materially  to  affect  the  nature  of  the  picture 
drawn  by  him.  In  the  drawings  of  Beer  and 
Madler,  Dawes,  and  other  astronomers,  as  in  the 
exquisite  pictures  constructed  by  Green  from  his 
study  of  the  planet  from  Madeira  during  its 
specially  favourable  appearance  in  1877,  the  planet, 
though  shown  in  greater  detail  and  perfection,  was 
essentially  the  Mars  of  Sir  William  Herschel,  and  his 
view  of  Mars  as  "a  miniature  of  the  Earth"  appeared 
to  derive  additional  confirmation.  More  recently, 
however,  interest  in  Mars  has  been  reawakened 
and  maintained  at  the  highest  pitch  by  the  alleged 
appearance  upon  the  surface  of  the  planet  of  a 
variety  of  detail  of  the  most  unexpected  and  per- 
plexing kind.  The  " canal  system"  of  Mars,  the 
first  suspicion  of  which  was  suggested  to  Schiapar- 
elli  in  1877,  if  it  has  led  to  speculations  that  scarcely 
add  to  the  dignity  of  science,  has  renewed  the  youth 
of  Martian  study,  and  has  directed  towards  the 


The  Recent  Study  of  Mars.  103 

planet  a  keen  scrutiny  only  rendered  possible  by 
the  construction  of  the  great  telescopes  of  modern 
times.  In  the  present  chapter  an  attempt  will  be 
made  to  trace  the  course  of  recent  discovery  upon 
Mars,  and  to  discuss,  though  necessarily  imper- 
fectly, certain  views  that  have  been  suggested,  and 
some  difficulties  that  have  arisen,  in  the  attempted 
interpretation  of  the  appearances. 

The  planet  Mars  lies  next  the  Earth  in  order  of 
increasing  distance  from  the  Sun,  the  distance 
of  Mars  being  141,500,000  miles,  while  that  of  the 
Earth  is  rather  less  than  93,000,000.  The  respec- 
tive distances  are  therefore  in  the  proportion  of 
1*523  to  i,  or  nearly  of  3  to  2,  a  relation  that 
will  be  useful  in  the  sequel.  The  diameter  of 
Mars  is  4230  miles,  that  of  the  Earth  being  7918 
miles,  from  which  it  follows,  from  simple  geometry, 
that  in  volume  the  Earth  exceeds  Mars  by  6*57 
times.  From  disturbances  produced  by  Mars  in 
the  movements  of  other  members  of  the  Solar 
System,  by  its  gravitational  attraction  upon  them, 
it  appears  that  its  mass — or  quantity  of  contained 
matter — is  less  than  that  of  the  Earth  in  the  pro- 
portion of  i  to  9*34.  Consequently,  the  Earth 
being  9*34  times  as  massive  while  only  6*57  times 
as  bulky  as  Mars,  the  density — or  mass  of  a  given 
bulk — of  the  Earth  must  exceed  that  of  Mars  in 
the  proportion  of  9*34  to  6*57,  or  of  1*42  to  i.  It 
also  follows  from  these  data  that  the  intensity  of 
gravitation  exercised  by  Mars  upon  bodies  at  its 
surface  must  be  less  than  that  exercised  by  the 
Earth  upon  bodies  at  its  surface  in  the  proportion 


104          Recent  Advances  in  Astronomy. 

very  nearly  of  2  to  5,  that  is,  a  body  the  weight  of 
which  had  been  determined  at  the  -surface  of  the 
Earth,  would,  if  it  were  transferred  to  the  surface  of 
Mars,  weigh  only  two-fifths  as  much.  Like  the 
Earth,  Mars  travels  round  the  Sun  in  an  orbit  that 
is,  although  nearly  circular,  slightly  elliptical,  the 
planes  of  the  two  orbits  very  nearly  coinciding. 
The  period  occupied  by  the  planet  in  completing 
its  orbit — that  is,  the  year  of  the  planet — is  686*9 
days,  the  Earth's  year  being  365*26  days.  The 
longer  period  of  Mars  is  due,  partly  to  the  greater 
length  of  its  orbit,  and  partly  to  the  planet's  speed 
in  its  orbit  being  less  than  that  of  the  Earth  in  its 
orbit,  a  consequence  of  its  greater  distance  from  the 
Sun. 

Like  all  planets  the  orbits  of  which  inclose  that 
of  the  Earth,  Mars  is  seen  to  best  advantage  when 
in  "  opposition  "  to  the  Sun — that  is,  at  the  instant 
at  which  the  Earth,  overtaking  the  planet  in  its 
slower  journey,  passes  directly  between  it  and  the 
Sun.  Under  these  conditions,  not  only  is  the  dis- 
tance separating  the  Earth  from  Mars  less,  and  the 
apparent  size  of  the  planet  therefore  greater,  than 
at  other  times;  but  the  hemisphere  of  the  planet 
that  is  illuminated  by  the  Sun  is  presented  directly 
towards  the  Earth,  so  that  the  disc  appears  "full ". 
At  other  times,  when  the  illuminated  hemisphere 
is  not  directly  presented  to  the  Earth,  the  planet 
exhibits  phases  resembling  those  of  the  Moon  when 
not  far  from  the  full.  The  phases  of  Mars  show 
that,  like  the  Earth  and  Moon,  it  is  not  inherently 
luminous,  but  that  it  is  rendered  visible  by  sunlight 


The  Recent  Study  of  Mars.  105 

scattered  from  its  surface.  It  is  clear  that,  as  is 
the  case  with  the  Moon  when  full,  a  planet  in 
opposition  to  the  Sun  must  rise  in  the  east  as  the 
Sun  sets  in  the  west,  and,  after  ascending  the 
heavens  during  the  evening  hours  and  attaining 
its  greatest  altitude  in  the  south  at  midnight,  must 
descend  towards  the  west  in  the  early  morning, 
setting  at  sunrise.  Hence,  a  further  advantage  of 
a  planet's  being  in  opposition  arises  from  its  being 
then  visible  through  all  the  hours  of  the  night. 

If  the  orbits  of  the  Earth  and  Mars  were  circles 
lying  in  the  same  plane  and  having  the  same  centre, 
and  if  the  Sun  occupied  the  common  centre,  then, 
at  every  opposition,  no  matter  what  position  in  its 
orbit  the  Earth  might  happen  to  occupy,  its  distance 
from  Mars  would  be  the  same.  Under  such  circum- 
stances Mars  would  appear  under  the  same  con- 
ditions at  every  opposition,  and  all  would  therefore 
be  equally  favourable.  These  simple  conditions  do 
not,  however,  exist.  The  planes  of  the  orbits  are 
indeed  so  nearly  coincident  that  their  deviation  from 
perfect  coincidence  may  for  the  present  purpose  be 
ignored;  but  the  forms  of  both  orbits  are  ellipses, 
deviating,  especially  in  the  case  of  the  orbit  of  Mars, 
appreciably  from  the  circular  form ;  while,  in  accord- 
ance with  Kepler's  first  law  of  planetary  motion,  the 
Sun  is  situated,  not  in  the  centre,  but  in  a  focus 
common  to  each  ellipse.  The  orbits  of  the  Earth 
and  Mars,  and  the  Sun's  position  relatively  to  them, 
are  represented  to  scale  in  the  accompanying  figure 
(fig.  6),  and  it  is  interesting  to  notice  that  the  ellip- 
ticity  of  each  orbit  is  indicated  far  more  clearly  in 


io6          Recent  Advances  in  Astronomy. 

the  displacement  of  the  Sun  from  the  centre  than  by 
deviation  from  circularity  in  outline,  which  indeed, 
even  in  the  case  of  the  more  elliptical  orbit  of  Mars, 
is  probably  inappreciable  to  the  most  critical  eye. 


1901  Feb.  22. 


o 


o 

T888  April  11. 


O 


(     J 1890  May  27. 
Fig.  6. — Oppositions  of  Mars. 


7892  August  3. 


From  the  eccentricities  of  the  orbits  of  the  Earth 
and  Mars,  and  from  the  position  of  the  Sun  rela- 
tively to  them,  it  follows  that,  in  one  direction — 
indicated  by  the  line  sx  in  the  figure — the  distance 
between  the  orbits,  as  measured  along  a  straight 
line  radiating  from  the  Sun,  is  least.  If,  therefore, 
at  the  time  that  the  Earth  is  crossing  this  line,  Mars 
also  happens  to  lie  upon  it,  an  opposition  will  result 


The  Recent  Study  of  Mars.  107 

which  will  be  the  most  favourable  possible;  the 
planet  then  appearing  brighter  to  the  naked  eye, 
and  presenting  a  larger  disc  when  viewed  through 
the  telescope,  than  at  any  other  time;  while  other 
oppositions  will  be  more  or  less  favourable  accord- 
ing as  to  whether  the  direction  of  the  Earth  and 
Mars  as  viewed  from  the  Sun  is  nearer  or  farther 
from  the  line  of  most  favourable  opposition  sx. 
The  Earth  in  its  annual  journey  round  the  Sun 
crosses  the  line  sx  upon  the  26th  of  August  in  each 
year;  hence,  the  nearer  to  this  date  of  occurrence, 
the  more  favourable  is  an  opposition  of  Mars. 

Since  the  periods  of  revolution  of  the  Earth  and 
Mars  round  the  Sun  are  365*26  and  686*9  days 
respectively,  it  follows,  from  simple  arithmetic, 
that,  upon  the  average,  the  Earth  must  overtake 
Mars,  and  an  opposition  must  therefore  occur,  at 
intervals  of  780  days,  or  nearly  two  years  and  two 
months.  This  would  be  the  constant  interval  be- 
tween any  two  successive  oppositions  if  the  orbits 
were  circles  with  the  Sun  in  their  common  centre, 
and  if,  as  would  then  necessarily  be  the  case,  the 
speed  of  each  planet  were  uniform.  Owing,  how- 
ever, to  the  elliptical  forms  of  the  orbits,  and  to  the 
fact,  expressed  in  Kepler's  second  law  of  planetary 
motion,  that  the  velocities  of  both  the  Earth  and 
planet  vary  with  their  distance  from  the  Sun,  the 
intervals  between  successive  oppositions  are  some- 
times greater  and  at  other  times  less  than  the  aver- 
age, the  exact  calculation  for  particular  cases  being 
a  very  laborious  matter.  In  the  figure  the  positions 
of  the  Earth  and  Mars  are  given  for  all  oppositions 


io8          Recent  Advances  in  Astronomy. 

occurring  between  1886  and  1901.  It  will  be  seen 
that  oppositions  are  now  (1898)  becoming  less  and 
less  favourable,  and  that  they  will  continue  to 
deteriorate  until  1901,  in  which  year  an  opposition 
will  occur  under  almost  the  most  unfavourable  con- 
ditions possible.  After  1901,  however,  improve- 
ment will  set  in,  culminating  in  fine  oppositions  in 
1907  and  1909.  The  circular  discs  arranged  outside 
the  orbit  of  Mars  in  the  figure  represent  to  scale  the 
relative  apparent  sizes  of  the  disc  of  Mars  as  seen 
from  the  Earth  at  the  different  oppositions.  They 
show  in  a  striking  manner  the  special  advantages 
attending  oppositions  that  occur  in  the  early  autumn 
months. 

Viewed  through  a  fine  telescope  and  under  favour- 
able conditions,  Mars,  when  in  opposition,  presents 
a  picture  of  singular  beauty  and  charm.  Markings, 
some  so  distinct  as  to  be  clearly  recognized  at  a 
first  glance,  others  less  strongly  pronounced,  and 
others  again  so  faint  as  to  tax  the  powers  of  the 
keenest  vision  assisted  by  the  finest  optical  power, 
are  distributed  over  the  disc-like  picture;  while  the 
beauty  of  the  spectacle  is  enhanced  by  the  presence 
and  variety  of  colour,  and  by  exquisite  gradations 
of  tint  in  different  regions. 

Upon  continuing  the  study  of  the  planet,  it  soon 
becomes  evident  that  change  is  in  progress,  not  in 
the  form  of  the  features  themselves,  but  in  their 
positions  relatively  to  the  outline  of  the  planet. 
Details  first  seen  near  the  centre  of  the  disc  have 
drifted  to  the  left;  others,  originally  near  the  left 
limb,  have  disappeared;  while  others,  previously 


The  Recent  Study  of  Mars.  109 

invisible,  have  appeared  within  the  right-hand  limb. 
These  changes  clearly  indicate,  that,  like  the  Earth, 
Mars  is  in  rotation.  Further,  it  becomes  apparent 
that  the  period  of  rotation  of  the  planet  does  not 
differ  very  much  from  that  of  the  Earth,  for  in  little 
more  than  twenty-four  hours  the  picture  presented 
is  again  that  originally  seen.1  Day  and  night,  the 
appearance  of  diurnal  revolution  of  the  heavens,  as 
well  as  all  other  celestial  phenomena  resulting  from 
the  rotation  of  a  planet,  follow  therefore  upon  Mars 
with  the  same  regularity,  and  at  nearly  the  same 
rate,  as  upon  the  Earth. 

From  the  study  of  the  planet's  rotation  it  is  a 
simple  matter  to  determine  the  position  within  it  of 
the  axis  about  which  the  rotation  takes  place. 
When  this  is  done,  it  is  found,  that,  as  is  the  case 
with  the  Earth,  the  axis  of  rotation  of  the  planet 
is  inclined  to  the  plane  of  its  orbit  round  the  Sun, 
the  inclination  of  the  axis  (24°  50')  curiously  approxi- 
mating in  value  to  that  of  the  inclination  of  the 
Earth's  axis  to  the  plane  of  its  orbit  (23°  27').  The 
tilt  of  the  axis  of  rotation  gives  rise  to  the  phenomena 
of  seasons;  hence  upon  Mars,  spring,  summer, 
autumn,  and  winter  follow  with  the  unceasing 
regularity  familiar  to  inhabitants  of  the  Earth. 

A  further  point  of  similarity  between  the  physical 
conditions  existing  upon  Mars  and  those  upon  the 
Earth  becomes  apparent  from  the  study  of  the 
planet's  rotation.  As  different  features  are  carried 
by  the  rotation  towards  the  left-hand  limb,  they 
disappear  while  still  at  an  appreciable  distance  from 

1  The  period  of  rotation  of  Mars  is  24  hours  37  minutes  23  seconds. 


no          Recent  Advances  in  Astronomy. 

it,  melting  into  a  luminous  ring  known  as  the 
"  limb-light ",  that  appears  to  continually  cling  to 
the  outline  of  the  planet,  extending  inwards  for 
some  distance  from  the  limb.  In  a  similar  manner, 
features  brought  into  view  by  rotation  do  not  at 
once  appear  as  they  are  brought  on  to  the  disc,  but 
as  gradually  emerge  from  the  limb-light  upon  their 
side  of  the  planet,  only  becoming  distinctly  visible 
when  the  rotation  has  carried  them  a  considerable 
distance  on  to  the  disc. 

The  suggestion  that  Mars  is  enveloped  in  an 
atmosphere  similar  in  its  physical  properties  to 
that  of  the  Earth  offers  so  simple  and  sufficient  an 
explanation  of  the  limb-light  that  it  is  scarcely 
possible  not  to  regard  it  as  the  true  one,  more 
especially  as  the  existence  of  an  atmosphere  upon 
Mars  is  independently  demonstrated  from  the 
nature  of  changes  continually  in  progress  in  the 
visible  features  of  the  planet,  to  which  attention  will 
shortly  be  directed.  According  to  this  view,  the 
appearance  of  the  limb-light  results  from  the 
scattering  of  the  Sun's  rays  in  the  atmosphere  of 
Mars.  Simple  considerations,  such  as,  for  instance, 
the  darkened  tint  of  the  sky  when  viewed  from 
high  altitudes,  indicate  that  the  appearance  of 
the  sky  as  a  vault  of  deep-blue  eternally  extended 
overhead  is  due  to  a  scattering  of  the  Sun's  rays 
in  the  atmosphere  of  the  Earth.  Tyndall  has 
shown  by  a  series  of  experiments  of  extreme 
beauty  that  this  scattering  is  in  all  probability 
effected  by  innumerable  minute  vesicles  of  water 
floating  in  the  atmosphere,  and  that  the  forma- 


The  Recent  Study  of  Mars.  in 

tion  of  these  vesicles  is  assisted  by,  or  may 
indeed  be  entirely  dependent  upon,  the  presence 
of  specks  of  dust,  which  form  nuclei  around  which 
condensation  of  the  vapour  of  water  present  in 
the  atmosphere  takes  place.  It  is  in  harmony 
alike  with  theory  and  experiment  that  the  more 
refrangible  of  the  Sun's  rays  should  experience 
such  scattering  to  a  far  greater  degree  than  those 
less  refrangible ;  so  that,  of  the  component  colours 
of  a  ray  of  sunlight  penetrating  a  column  of  atmos- 
phere, the  more  refrangible  colours — those  near 
the  violet  end  of  the  spectrum — should  be  scattered 
in  all  directions  around,  while  the  less  refrangible — 
the  red  and  adjacent  rays  of  the  spectrum — should 
pass  through  more  readily,  a  law  illustrated  in  the 
familiar  fact  of  the  great  penetrative  power  of  a  red 
light  in  a  fog,  and  also  supplying  an  explanation  of 
the  transparent  blue  of  the  noonday  sky  and  the 
crimson  colours  of  sunset.  Assuming  the  existence 
upon  Mars  of  an  atmosphere  possessing  similar 
dispersive  powers,  the  limb-light  is  simply  and 
naturally  explained.  Bathed  in  the  Sun's  rays 
and  containing  floating  matter  capable  of  scatter- 
ing them,  the  atmosphere  of  Mars  would  form  a 
luminous  shell  enshrouding  the  visible  hemisphere 
of  the  planet.  The  line  of  vision  from  the  Earth 
directed  to  the  centre  of  the  disc  pierces  this  shell 
perpendicularly;  the  portion  of  its  length  included 
in  the  shell  is  therefore  the  least  possible,  and  the 
illumination  of  the  atmosphere  is  barely  appreciable. 
The  line  of  sight  to  the  limb,  however,  meets  the 
visible  hemisphere  tangentially,  and,  traversing  the 


ii2          Recent  Advances  in  Astronomy. 

air-shell  very  obliquely,  its  intercepted  length  is 
great,  and  the  illumination  of  the  air  very  apparent. 
From  the  edge  towards  the  centre  of  the  disc  the 
length  of  the  line  of  vision  involved  in  the  atmos- 
phere of  the  planet  continually  decreases,  the 
appearance  of  illumination  therefore  becomes  less 
and  less,  and  the  limb-light  is  the  result. 

The  fading  of  the  planetary  features  upon  ap- 
proaching the  limb  is  further  aided,  first,  by  the 
fact  that  as  they  approach  the  edge  of  the  visible 
hemisphere  their  actual  illumination  becomes  less, 
as  does  the  terrestrial  landscape  towards  sunset, 
both  from  the  increasing  slant  of  the  Sun's  rays  and 
by  the  greater  absorption  exercised  upon  the  rays 
from  the  greater  length  of  their  atmospheric  path;1 
and,  secondly,  from  the  greater  length  of  the 
Martian  atmosphere  through  which  they  are  viewed, 
and  the  consequent  increased  absorption  exercised 
upon  the  rays  in  retraversing  the  atmosphere,  after 
reflection  from  the  surface  of  the  planet. 

Explanations  other  than  the  one  advanced  here 
have  been  suggested  to  explain  the  limb-light. 
The  appearance  has  been  ascribed  to  the  deposition 
of  hoar-frost,  upon  the  approach  of  night,  over 
regions  about  to  enter  the  dark  hemisphere  of  the 
planet;  and  to  the  lingering  of  the  frost  in  the  early 
morning  over  those  that  have  recently  emerged 
from  it.  This  suggestion  appears,  however,  to  be 
disproved  by  the  observed  symmetry  of  the  limb- 
light  in  the  cold  polar  and  warmer  equatorial 
regions  of  the  planet;  and  by  the  fact  that,  in 

1  This  only  applies  to  Mars  when  in  or  near  opposition. 


The  Recent  Study  of  Mars.  113 

observations  made  at  times  when  Mars  is  not  in 
opposition,  and  when,  consequently,  its  disc  does 
not  appear  to  be  full,  the  limb-light  has  been  seen 
to  cling  to  the  limb  itself  in  preference  to  those 
regions  in  which  morning  and  evening  are  in- 
dicated by  proximity  to  the  terminating  line  sepa- 
rating the  dark  from  the  bright  hemisphere. 

The  tenuity  of  the  veil  spread  over  the  illuminated 
hemisphere  by  the  atmosphere  is  generally  taken  to 
indicate  that  the  surface  density  of  the  atmosphere 
— that  is,  the  quantity  of  air  accumulated  over  each 
square  mile  of  surface — is  less  in  the  case  of  Mars 
than  in  that  of  the  Earth.  It  is  probable,  that  if 
the  surface  density  of  the  Martian  atmosphere  were 
equal  to  that  of  the  Earth,  its  veiling  effects  would 
be  far  more  pronounced  than  they  are;  and  that, 
even  in  the  centre  of  the  disc,  the  surface  markings 
would  be  permanently  concealed  beneath  a  brilliant 
haze.  Recognizing  that  the  appearance  of  the  sky 
is  the  result  of  the  scattering  of  sunlight  in  the 
Earth's  atmosphere,  it  will  be  apparent  that,  to 
an  observer  who  should  ascend  above  the  highest 
reaches  of  the  air,  the  Earth  would  appear  upon  a 
clear  day  to  be  veiled  by  the  blue  haze  of  the  sky, 
now  lying  between  him  and  the  landscape  beneath. 
From  actual  measurements  of  the  brightness  of 
the  sky  carried  out  by  Langley  it  has  been  con- 
cluded that  to  such  an  observer  all  terrestrial 
features  except  the  most  brilliant  would  be  scarcely 
visible,  their  fainter  light  being  overwhelmed  by  the 
more  intense  glare  of  the  intervening  atmosphere. 
To  a  possible  inhabitant  of  another  planet,  provided 

(M520)  H 


ii4          Recent  Advances  in  Astronomy. 

with  adequate  instrumental  means,  the  Earth  would 
appear  as  a  dazzlingly  brilliant,  but  probably  a 
nearly  uniformly  illuminated  orb.  The  ice-bound 
regions  near  the  poles  and  the  snow-clad  summit  of 
a  mountain,  might,  here  and  there,  be  clearly  dis- 
tinguishable in  the  general  luminosity  of  its  disc, 
but  it  would  scarcely  be  possible  to  trace  upon  it 
any  of  the  more  familiar  terrestrial  features.  Upon 
Mars,  however,  surface  markings  are  clearly  re- 
cognized unless  fairly  close  to  the  limb,  while  those 
in  the  centre  of  the  disc  appear  to  experience  but 
slightly  the  effects  of  atmospheric  veiling.  It  is 
therefore  commonly  assumed  that  in  the  density  of 
surface  distribution  of  atmosphere,  Mars  is  poorer 
than  the  planet  Earth. 

It  must  be  acknowledged  that  this  reasoning, 
though  lending  a  strong  probability  to  the  view,  is 
not  quite  conclusive.  It  essentially  rests  upon  the 
assumption  that  the  power  of  an  atmosphere  to 
scatter  light  may  be  taken  as  a  measure  of  its 
density.  It  has  been  seen,  however,  that  the 
scattering  of  light  is  effected  by  solid  and  liquid 
matter  suspended  in  the  air,  and  is  not,  therefore, 
an  inherent  property  of  an  atmosphere  itself.  Were 
there  no  floating  matter  in  the  Earth's  atmosphere, 
there  would  be  no  scattering  of  light  within  it.  The 
blue  sky,  even  in  the  immediate  neighbourhood  of 
the  noonday  sun,  would  under  such  conditions  be 
replaced  by  a  vault  of  intense  black,  in  which,  by 
day  as  by  night,  the  stars  would  shine  with  a  lustre 
unknown  even  on  the  clearest  and  darkest  nights. 
Absorption  of  light  would  still  occur,  but  to  so 


The  Recent  Study  of  Mars  115 

slight  an  extent — such  is  the  transparency  of  pure 
air — as  to  be  barely  appreciable ;  while  it  is  hardly 
necessary  to  state  that  the  absorbed  light  would  be 
entirely  extinguished,  and  that  no  appearance  of  a 
sky  could  result  from  it.  That  the  atmosphere  of 
Mars  contains  floating  matter  in  proportion  to  its 
density  may  be  true,  but  it  is  an  assumption  that  it 
is  not  possible  to  verify.  It  will  be  seen  later  that 
there  are  indications  of  a  scarcity  of  water  on  the 
surface  of  Mars,  and  that  there  is  a  very  strong 
probability  that  its  atmosphere  is  charged  with  the 
vapour  of  water  to  a  far  less  extent  than  is  the 
atmosphere  of  the  Earth.  It  is  probable  that  the 
condensation  of  the  vapour  of  water  plays  an  im- 
portant part  in  the  dispersive  action  of  the  atmos- 
phere on  light,  and  that,  therefore,  under  conditions 
otherwise  similar,  a  less  moist  atmosphere  would 
possess  a  feebler  scattering  power  than  another. 
That  Mars  possesses  a  more  tenuous  atmosphere 
than  that  of  the  Earth  may  be  probable,  but  an 
equal  or  even  a  greater  density  is  not  inconsistent 
with  the  telescopic  aspect  of  the  planet.1 

Spectroscopic  evidence  bearing  upon  the  question 
of  the  Martian  atmosphere  is  so  curiously  conflicting 
that  it  is  perhaps  better  to  wait  for  further  observa- 
tions before  taking  it  seriously  into  account. 

Of  the  different  features  apparent  upon  the  disc  of 
Mars,  generally  the  most  conspicuous  are  two  white 
patches,  nearly  always  visible  in  the  neighbourhood 

1  The  a  priori  arguments,  based  upon  the  relative  volumes  and  masses  of 
Mars  and  the  Earth,  that  are  frequently  adduced  as  evidence  for  a  rare 
atmosphere  on  the  planet  appear  to  possess  little  if  any  value. 


u6          Recent  Advances  in  Astronomy. 

of  the  poles,  though  not  arranged  symmetrically 
round  them.  So  brilliant  are  they,  that  they  have 
been  seen  sparkling  like  twin  stars  at  times  when 
the  sky  has  been  covered  by  haze  to  such  an  extent 
that  the  outlines  of  the  planet  itself  have  been  in- 
visible. From  their  general  appearance,  as  well  as 
from  their  situation  in  the  immediate  neighbourhood 
of  the  poles,  they  have  been  regarded  as  accumula- 
tions of  snow  and  ice,  similar  in  their  nature  and  in 
their  mode  of  formation  to  the  polar  caps  of  the 
Earth.  This  conclusion  is  strongly  supported  by 
the  nature  of  the  changes  apparent  in  both  of  them 
during  the  progress  of  the  Martian  seasons.  Upon 
the  approach  and  during  the  continuance  of  winter 
in  either  hemisphere  of  Mars,  as,  in  the  orbital 
movement  of  the  planet,  the  hemisphere  is  turned 
from  the  Sun,  the  white  cap  surrounding  its  pole 
continually  increases,  its  boundaries  extending 
farther  and  farther  towards  the  equator;  while  later, 
during  spring  and  summer,  as  the  hemisphere  is 
again  turned  towards  the  Sun,  its  white  covering 
dwindles  in  dimension,  becoming  generally  reduced 
to  an  insignificant  oval  patch.  Upon  a  recent  occa- 
sion, indeed,  when  the  planet  was  near  its  opposition 
in  1894,  the  south  polar  cap  entirely  vanished,  the 
substance  composing  it  having  been  apparently 
dissipated  beneath  the  rays  of  the  summer  sun. 
Other  appearances,  more  rarely  recognized  in  the 
caps  and  in  their  immediate  neighbourhood,  lend 
additional  support  to  this  view.  On  June  8th,  1894, 
Mr.  Lowell,  while  observing  Mars  from  Arizona, 
saw  two  points  of  light  of  dazzling  brilliancy  flash 


The  Recent  Study  of  Mars.  117 

out  in  the  midst  of  the  south  polar  cap.  For  a  few 
moments  they  sparkled  in  the  surrounding  white- 
ness and  then  disappeared.  It  is  difficult  to  resist 
Mr.  Lowell's  interpretation  that  their  appearance  was 
due  to  the  glint  of  ice-slopes  flashing  the  sunlight 
towards  the  Earth,  as,  during  the  rotation  of  the 
planet,  the  slopes  were  for  a  few  moments  placed  at 
the  proper  angle  to  the  rays.  Similar  appearances 
had  been  noticed  by  Mr.  Green  during  the  opposi- 
tion of  1877,  but  in  this  case  they  were  seen  near  to, 
but  not  actually  involved  in,  the  polar  cap. 

Distributed  round  the  planet  in  a  rough  zone  ap- 
preciably parallel  to  its  equator,  and  extending  over 
considerably  more  than  a  half  of  its  entire  surface, 
are  a  number  of  patches,  generally  of  a  soft  rounded 
outline,  and  of  a  colour  that  has  suggested  the 
orange-yellow  of  a  field  of  ripe  corn.  It  is  to  these 
that  the  planet  owes  the  familiar  ruddy  tint  that  has 
caused  it  to  be  associated  in  name  with  the  god  of 
war.  Bounding,  and  frequently  deeply  indenting, 
these  orange  masses  are  regions  of  a  gray-green 
tint,  and  these  complete  the  picture  of  the  planet's 
surface  as  seen  with  moderate  optical  power.  En- 
couraged by  the  close  similarity  in  appearance  and 
behaviour  between  the  polar  caps  of  Mars  and  the 
Earth,  it  has  been  the  custom  to  pursue  the  analogy 
further,  and  to  see  in  the  orange  patches  and  in  the 
gray-green  markings  upon  Mars  the  continents  and 
oceans  of  a  miniature  world. 

That  the  orange  masses  upon  Mars  are  indeed 
land  appears  probable,  from  the  similarity  in  their 
appearance  to  that  which  it  may  be  well  supposed 


n8          Recent  Advances  in  Astronomy. 

the  great  deserts  of  the  Earth  would  .present  under 
similar  conditions  of  observation,  as  well  as  from 
the  permanent  appearance  upon  them  of  delicate 
markings  revealed  by  higher  optical  power;  though, 
perhaps,  as  strong  an  argument  as  any  lies  in  the 
difficulty  of  suggesting  any  other  explanation  for 
their  appearance.  That  the  gray-green  markings 
are  the  surfaces  of  Martian  seas  appears  at  a  first 
glance  a  scarcely  less  plausible  suggestion.  Their 
colour  is  not  unlike  that  of  water ;  the  existence  of 
extensive  tracts  of  water  upon  Mars  harmonizes  well 
with  the  view  of  the  polar  caps  as  accumulations  of 
snow  and  ice;  and  their  aqueous  character  was  sup- 
posed to  have  received  its  final  confirmation  in  1867, 
from  the  announcement  of  Sir  William  Huggins, 
that,  from  the  spectroscopic  examination  of  the  light 
from  Mars,  he  had  detected  the  existence  of  the 
vapour  of  water  as  a  constituent  of  its  atmosphere. 
Of  late  years,  however,  considerable  doubt  has  been 
thrown  upon  this  rather  attractive  view.  In  1877 
Schiaparelli  of  Milan  maintained  that,  if  the  gray- 
green  markings  were  the  surfaces  of  water,  they 
should  occasionally,  when  turned  at  the  proper 
angle  to  the  directions  of  the  Sun  and  Earth,  unless 
indeed  their  surfaces  were  continually  in  a  state  of 
violent  disturbance,  reflect  the  Sun's  rays  in  such  a 
manner  that  its  image  should  appear  as  a  bright 
star  sparkling  upon  them.  It  is  not  difficult,  upon 
the  assumption  that  the  water  surface  is  clean  and 
still,  to  calculate  the  intensity  of  the  solar  image 
that  should  be  formed  under  the  actual  conditions, 
and  it  appears  that  it  should  be  so  brilliant  as  to  be 


The  Recent  Study  of  Mars.  119 

readily  capable  of  recognition.  No  such  appear- 
ance has,  however,  ever  been  recorded  upon  the 
disc  of  Mars. 

A  very  interesting  though  not  in  itself  a  con- 
clusive observation  has  recently  been  made  by  Pro- 
fessor W.  H.  Pickering,  in  the  examination  of  Mars 
under  the  polariscope.  It  is  well  known  that  the 
fraction  of  light  that  is  regularly  reflected  from  the 
surface  of  any  transparent  substance  exhibits  the 
phenomena  of  polarization — it  is  capable  of  being 
again  reflected  more  or  less  effectively  by  a  second 
transparent  surface,  according  to  its  direction  of 
incidence  upon  it;  it  is  transmitted  through  certain 
crystals — such  as  tourmaline — more  or  less  readily, 
according  to  the  direction  of  the  axis  of  the  crystal ; 
and  upon  traversing  many  crystals,  and  in  its 
subsequent  analysis  by  a  second  polarizing  appa- 
ratus, it  is  capable  of  developing  the  exquisite  effects 
of  colour  familiar  to  many  observers  with  the  micro- 
scope. It  is  a  simple  matter  to  detect  polarization 
in  light  reflected  from  a  plane  glass  or  a  water  sur- 
face, especially  for  certain  angles  of  incidence,  and 
in  the  light  of  the  sky,  which  is  strongly  polarized 
as  the  result  of  its  scattering  by  water  vesicles 
suspended  in  the  atmosphere.  In  1894  Pickering 
examined  the  light  from  the  gray-green  markings 
upon  Mars  with  a  specially-constructed  polariscope, 
but  failed  to  detect  in  any  of  them  any  trace  of 
polarization. 

There  is  no  doubt  that  if  polarization  had  been 
evident  in  the  light  of  the  gray-green  markings, 
their  liquid  nature  would  have  been  demonstrated ; 


i2o          Recent  Advances  in  Astronomy. 

but  the  interpretation  of  the  negative  evidence  is  not 
so  definite.  Polarization  would  be  produced  by 
regular  reflection — by  which  is  meant  reflection  as 
from  a  mirror,  the  angle  of  incidence  being  equal  to 
that  of  reflection — or  it  might  conceivably  result 
from  the  scattering  of  light  by  fine  particles  sus- 
pended in  a  body  of  water,  in  a  manner  analogous 
to  that  by  which  the  appearance  of  the  sky  is  pro- 
duced. If,  however,  the  surface  of  water  were  not 
clean,  the  impurities  upon  it  would  scatter  light 
incident  upon  it  in  all  directions,  and  in  light  so 
scattered  no  polarization  should  be  apparent;  there 
would,  in  fact,  be  no  water  surface  exposed  to  the 
light,  but  a  dirt  surface  concealing  a  water  surface 
beneath.  Pickering's  observations  would  appear  to 
indicate  that  if  the  aqueous  view  of  the  gray-green 
markings  is  to  be  retained,  it  must  be  modified  in 
this  direction.  The  same  modification  would  also 
account  for  the  non-appearance  of  the  image  of  the 
Sun  upon  the  surfaces  of  Martian  seas. 

Still  more  recent  and  direct  observations  appear 
to  involve  a  complete  refutation  of  the  aqueous 
character  of  the  gray- green  markings  on  Mars. 
Mr.  Douglass  at  Arizona  in  1894,  an(^  Mr.  Barnard 
at  the  Lick  Observatory  in  1896,  have  succeeded  in 
distinguishing  over  the  entire  surfaces  of  them  a 
considerable  amount  of  delicate  and  permanent 
detail,  an  intricate  tracery  clearly  inconsistent  with 
the  older  view  as  to  their  nature.  Mr.  Barnard,  in 
particular,  examining  the  planet  with  the  superb 
refractor  of  the  Lick  Observatory  upon  Mount 
Hamilton,  under  atmospheric  conditions  that  fre- 


The  Recent  Study  of  Mars.  121 

quently  approximated  to  perfection,  describes  the 
detail  revealed  in  the  regions  of  the  so-called  seas 
as  being  so  intricate,  small,  and  abundant,  that  it 
baffled  all  attempts  to  properly  delineate  it.  He 
suggests  that,  to  those  who  have  looked  down  upon 
a  mountainous  country  from  a  considerable  eleva- 
tion, some  conception  of  the  appearance  presented 
may  be  formed.  From  the  appearance  of  the 
country  round  Mount  Hamilton  as  seen  from  the 
observatory,  it  was  possible  to  imagine  that,  as 
viewed  from  a  great  altitude,  this  region,  broken  by 
canon,  slope,  and  ridge,  would  closely  resemble 
the  surface  of  the  Martian  seas.  During  the  obser- 
vations the  conviction  seemed  to  force  itself  upon 
the  observer  that  he  was  actually  looking  down 
from  a  great  elevation  upon  just  such  a  surface 
as  that  above  which  the  observatory  was  situated. 

It  appears,  therefore,  that  if  water  exists  at  all 
upon  Mars  in  the  liquid  form,  it  must  be  sought 
elsewhere  than  in  the  so-called  seas;  and  it  is 
possible  that,  in  an  observation  made  in  1894  by 
Mr.  Lowell  and  Professor  Pickering,  its  place  upon 
the  surface  of  the  planet  was  revealed  for  the  first 
time.  During  a  careful  study  of  Mars,  when  near 
its  opposition  in  that  year,  with  the  aid  of  a  fine 
refracting  telescope  of  18  inches  of  aperture,  there 
appeared  a  dark  belt  forming  a  fringe  to  the  south 
polar  cap.  The  belt  first  appeared  after  rather 
more  than  a  Martian  month  following  the  spring 
equinox  of  the  planet.  It  was  estimated  as  being 
the  darkest  marking  on  the  disc,  and  appeared  to 
be  of  a  decidedly  blue  colour.  As  the  polar  cap 


122          Recent  Advances  in  Astronomy. 

dwindled,  the  belt  followed,  clinging  to  its  edge. 
At  midsummer  upon  Mars  it  was  described  as  a 
barely  discernible  thread  drawn  round  the  minute 
white  patch,  which  was  all  that  then  remained  of 
the  enormous  snow-fields  of  some  months  before. 
Finally,  when  the  cap  vanished,  the  spot,  where 
its  girdle,  long  since  too  small  for  detection,  had 
existed,  had  become  one  yellow  stretch.1 

That  the  belt  seen  upon  this  occasion  was  water, 
or  at  any  rate  liquid  formed  by  the  melting  of  the 
polar  cap,  appears  a  plausible  suggestion,  and 
appears  more  probable  from  the  fact  that  Professor 
Pickering,  on  subjecting  it  to  examination  with 
the  polariscope,  was  convinced  that  its  light  showed 
marked  evidence  of  polarization.  The  interpreta- 
tion of  the  sequence  of  the  observed  phenomena 
appears  to  be — that  the  melting  of  the  polar  cap 
gave  rise  to  a  fringing  belt  of  liquid,  which  first 
appeared  as  such,  but  was  rapidly  distributed  over 
the  summer  hemisphere  in  streams  too  fine  for 
detection. 

A  dark  belt  surrounding  the  north  polar  cap  had 
been  seen  as  early  as  1830  by  Beer  and  Madler, 
and  other  like  appearances,  which  may  have  been 
of  the  same  nature,  have  been  recorded  by  other 
astronomers. 

The  atmosphere  of  Mars  appears  to  be  in  striking 
contrast  with  that  of  the  Earth,  in  its  almost  entire 
freedom  from  cloud  or  mist.  From  time  to  time 
during  the  study  of  the  planet,  extensive  regions 
have  appeared,  sometimes  for  a  considerable  time, 

1  Mars,  by  Percival  Lowell. 


The  Recent  Study  of  Mars.  123 

to  be  indistinct,  the  result,  it  has  been  generally 
supposed,  of  accumulated  cloud  or  mist;  but  as 
more  perfect  optical  means  have  been  applied,  and 
as  observations  have  been  conducted  from  localities 
specially  selected  for  their  atmospheric  steadiness 
and  the  consequent  improvement  in  the  definition 
of  the  telescopic  picture  the  appearances  have 
become  less  and  less  frequent.  During  the  entire 
course  of  a  series  of  observations  upon  the  planet, 
continually  maintained  at  Flagstaff  in  Arizona, 
under  the  direction  of  Mr.  Lowell,  from  May  to 
the  end  of  November,  in  the  year  1894,  no  case 
of  obscuration  that  could  be  ascribed  to  cloud  or 
mist  was  recorded  by  anyone  of  the  three  astrono- 
mers engaged  in  the  work,  with  the  exception, 
perhaps,  of  some  minute  white  specks,  limited  in 
position  to  the  immediate  neighbourhood  of  the 
line  of  division  between  the  bright  and  dark  hemi- 
spheres, and  which  may  have  been  transient  morning 
and  evening  clouds.  During  the  actual  progress 
of  these  observations,  however,  other  astronomers, 
observing  Mars  with  less  perfect  optical  means  and 
under  less  favoured  atmospheric  conditions,  believed 
that  they  recognized  one  of  the  most  extensive  for- 
mations of  cloud  that  has  ever  been  recorded.  To 
Mr.  Stanley  Williams  at  Brighton,  for  instance,  the 
greater  part  of  the  Miraldi  Sea,  one  of  the  largest, 
darkest,  most  definite,  and  most  characteristic  of  the 
green  regions,  disappeared  almost  entirely  from 
view,  apparently  densely  obscured  by  cloud  or  mist.1 
There  is  no  doubt  that  changes  in  the  tint  of  several 

1  Observatory,  1894,  p.  391. 


124          Recent  Advances  in  Astronomy. 

regions  of  the  planet's  surface  are  of  frequent  occur- 
rence, and  it  is  possible  that  such  changes,  which 
may  cause  the  disappearance  of  detail,  especially 
if  accompanied  by  a  general  lightening  of  tint, 
may  have  been  interpreted  as  cloud  and  mist.  In 
the  apparent  absence  of  cloud,  and,  consequently, 
of  rain,  upon  the  surface  of  the  planet,  it  is  probable 
that  the  polar  caps  are  formed  by  the  continued 
deposition,  as  hoar-frost,  during  the  long  Martian 
winter,  of  the  vapour  of  water  or  of  some  other 
liquid  present  in  the  atmosphere. 

When  the  planet  was  near  its  very  favourable 
opposition  in  1877,  Schiaparelli  at  Milan,  while 
observing  with  a  telescope  of  rather  over  8  inches 
in  aperture,  detected  certain  faint  dusky  lines  pro- 
jecting from  the  gray-green  regions  well  into  the 
interior  of  the  orange  continents.  The  streaks 
appeared  to  be  most  conspicuously  visible  shortly 
after  the  mid-winter  of  the  hemisphere  in  which 
they  appeared.  At  the  rather  less  favourable 
opposition  of  1879,  the  streaks  first  seen  were  traced, 
accompanied  by  others;  to  Schiaparelli  they  ap- 
peared to  be  more  sharply  defined  than  before; 
while  one  of  them  appeared  to  be  double,  consisting 
of  a  pair  of  parallel  streaks  separated  by  a  distance 
of  between  one  and  two  hundred  miles.  As  before, 
as  well  as  at  succeeding  oppositions,  the  streaks, 
to  which  Schiaparelli  had  now  given  the  most  un- 
fortunate name  of  "  canals",  appeared  more  clearly 
during  the  latter  part  of  the  Martian  winter  and 
the  early  spring.  At  succeeding  and  increasingly 
unfavourable  oppositions,  the  numbers,  length, 


The  Recent  Study  of  Mars.  125 

and  instances  of  duplication,  of  the  canals  were 
steadily  increased;  in  character  they  seemed  to  be 
more  rectilinear  and  more  sharply  defined;  and, 
to  Schiaparelli,  they  at  length  appeared  to  form 
a  reticulated  network,  extended  over  nearly  the 
whole  of  the  orange  continents.  Until  1896  to 
no  other  observer  had  the  canals  so  much  as  ap- 
peared, but  in  that  year  a  few  were  recognized 
by  Perrotin  and  Thollon  at  Nice,  by  the  aid  of  a 
then  newly  -  constructed  telescope  of  29  inches  of 
aperture.  As  oppositions  again  became  more 
favourable,  however,  they  appeared  to  quite  a 
number  of  astronomers  supplied  with  the  most 
ordinary  instrumental  means;  and  they  have  now 
become  entirely  notorious. 

According  to  the  evidence  of  astronomers  to 
whom  they  have  appeared,  the  canals  are  faint 
lines  that  appear  to  become  finer  and  straighter 
as  the  eye  becomes  accustomed  to  their  appearance. 
Their  width  is  estimated  as  being  not  less  than 
fifteen,  or  more  than  sixty,  miles.  They  follow, 
as  closely  as  can  be  seen,  the  course  of  great  circles 
upon  the  surface  of  the  planet,1  and  can  be  fre- 
quently traced  for  upwards  of  1500  miles.  They 
mutually  intersect  in  a  most  remarkable  manner, 
several  of  them  frequently  passing  through  the 
same  point,  from  which,  again,  it  is  not  uncommon 
to  find  other  canals  originating,  so  that  the  entire 

1  A  great  circle  of  a  sphere  is  the  circle  that  divides  it  into  two  equal  parts. 
It  is  the  largest  circle  that  can  be  drawn  upon  the  surface,  and  its  course 
marks  the  shortest  line  that  can  be  drawn  between  two  points  on  the  surface. 
Great  circles  are  illustrated  in  meridians  of  longitude.  Parallels  of  latitude 
are  known  as  small  circles. 


126          Recent  Advances  in  Astronomy. 

surface  of  the  planet  appears  as  if  involved  in  a 
complicated  network  of  delicate  tracery.  Before 
the  year  1894,  canals  had  only  been  recognized 
upon  the  orange  continents;  but  as  the  planet 
approached  opposition  in  that  year  they  appeared 
to  Mr.  Douglass  at  Flagstaff  to  be  distributed 
scarcely  less  richly  over  the  green  of  the  so-called 
seas.  Points  of  intersection  of  canals  are  frequently 
emphasized  by  the  occurrence  at  them  of  round  or 
oval  dusky  spots,  which  have  received  the  name  of 
"  lakes".  The  system  of  canals  and  their  associated 
lakes  varies  in  visibility  with  the  Martian  seasons, 
being  commonly  invisible  during  winter,  gradually 
appearing  in  the  early  spring,  and  again  disappear- 
ing during  the  progress  of  late  summer  and  autumn. 
It  has  been  suggested  that  the  canals  of  Mars 
are  waterways,  and  that  their  emergence  from 
invisibility  upon  the  approach  of  spring  may  be 
due,  either  to  the  dissipation  of  their  winter  cover- 
ing of  ice  and  snow,  or  from  their  becoming 
extensively  flooded  by  large  volumes  of  water  dis- 
charged into  them  by  the  melting  of  the  polar 
snows.  Their  light  revealed  no  trace  of  polarization 
when  examined  by  Professor  W.  H.  Pickering, 
from  which  he  has  advanced  the  conjecture  that 
the  streaks  may  be  tracts  of  country  lying  upon 
either  side  of  water-courses,  themselves  too  fine 
for  detection,  and  irrigated  by  them,  rather  than 
water  -  courses  themselves;  and  that  their  appear- 
ance in  the  spring  may  be  due  to  a  general  growth 
of  early  vegetation  over  them  as  they  become 
fertilized  by  the  flooding  of  the  streams. 


The  Recent  Study  of  Mars.  127 

The  apparent  regularity  of  the  canals,  as  well  as 
the  difficulty  of  suggesting  any  other  explanation 
for  them,  have  been  at  times  regarded  as  indicating 
artificiality.  According  to  Mr.  Lowell,  who  is  a 
strong  advocate  of  this  view,  the  canals  and  their 
connected  lakes,  which,  according  to  this  view, 
may  be  more  suitably  regarded  as  oases,  are  the 
visible  result  of  an  extensive  system  of  irrigation 
carried  out  by  intelligent  beings  on  Mars.  For  the 
inhabitants  of  Mars,  as  for  man,  water  is  a  necessity 
of  life;  and  since  water  appears  to  be  scarce  on  the 
planet,  being,  indeed,  apparently  only  to  be  ob- 
tained from  the  melting  of  the  polar  snows,  the 
inhabitants  have,  with  consummate  engineering 
skill,  constructed  an  extensive  network  of  channels, 
extending  from  the  polar  regions  over  the  entire 
surface  of  the  planet.  By  these  channels,  upon  the 
melting  of  the  polar  snow,  the  lower  lands  are  well 
supplied  with  water,  the  vegetation  springing  up 
on  them  giving  rise  to  the  appearance  of  the  gray- 
green  tracts ;  while  the  irrigation  of  the  higher  and 
desert  districts  is  confined  to  the  immediate  neigh- 
bourhood of  the  channels,  and  results  in  growth  of 
vegetation  over  belts  of  country  irrigated  by  them 
on  either  side,  and  the  oases  at  their  junctions. 
From  these  follow  the  appearances  of  the  "  canals" 
and  "  lakes". 

So  much  for  the  outlines  of  a  romance,  the  lead- 
ing leatures  of  which  have  become,  largely  through 
the  co-operation"  of  "our  own  correspondent"  and 
Mr.  Lowell,  familiar  to  the  greater  number  of 
readers  of  the  daily  press  about  the  times  of  recent 


128          Recent  Advances  in  Astronomy. 

oppositions  of  Mars.  To  those  not  practically  ac- 
quainted with  the  extreme  delicacy  involved  in  the 
telescopic  observation  of  detail  so  faint  as  just  to 
hover  upon  the  verge  of  the  visible  and  the  unsee- 
able, it  must  appear  that,  to  the  concurrent  testi- 
mony of  so  many  laborious  observations,  conducted 
by  astronomers,  some  of  established  reputation,  and 
the  greater  number  of  unquestioned  honesty  of 
purpose,  there  can  be  but  one  interpretation;  and 
that,  upon  the  surface  of  Mars,  features  and  a  sys- 
tem alike  unique  in  the  revelations  of  the  Universe 
have  been  firmly  established.  The  examination  of 
more  complete  evidence,  however,  suggests  grave 
objections  to  the  unhesitating  acceptance  of  this 
view,  while  to  many  thoughtful  observers  it  has 
appeared  hard  to  escape  from  the  conclusion  that 
the  complicated  meshes  of  the  canal  system  upon 
Mars  must  be  regarded  as  little  more  than  optical 
illusions,  faulty  interpretations  of  the  faintest  shades 
of  tint,  the  exact  nature  of  which  has  not  so  far 
been  established. 

Although  originally  discovered,  and  the  courses 
of  many  of  them  traced,  by  the  aid  of  a  telescope  of 
scarcely  more  than  8  inches  in  aperture;  although 
continually  seen  in  England  and  elsewhere  through 
instruments  of  still  less  power;  and  although,  by 
such  aid,  the  surface  of  Mars  has  been  mapped  by 
harsh  black  lines  in  a  manner  that  suggests  the 
transformation  of  a  world  into  a  gigantic  shunting- 
yard,  the  canals,  at  any  rate  in  their  generally 
assumed  characteristics,  have  consistently  refrained 
from  appearing  upon  the  picture  of  the  planet 


The  Recent  Study  of  Mars.  129 

formed  in  many  of  the  finest  telescopes  in  the 
world,  directed  by  astronomers  who,  in  other  and 
independent  work,  have  earned  the  highest  reputa- 
tion for  keenness  of  vision.  Through  the  Wash- 
ington refractor  of  26  inches  of  aperture,  the 
instrument  by  which,  in  1877,  Professor  Hall  first 
detected  the  moons  of  Mars,  the  canals  have  never 
been  traced.  Dr.  Keeler,  of  the  Alleghany  Obser- 
vatory, made  a  special  study  of  Mars  when  near  its 
opposition  in  1892  with  a  refractor  of  13  inches  of 
aperture.  During  the  course  of  the  observations 
the  definition  of  the  planetary  outlines  was  fre- 
quently so  excellent  that  the  moons  of  Mars  were 
clearly  visible  in  the  field  of  view;  but  although 
certain  ill-defined  shaded  streaks  were  recognized 
near  the  recorded  positions  of  canals,  no  trace  of 
their  hard  rectilinear  character,  or  of  their  marvel- 
lously reticulated  system,  was  detected.  Near  the 
time  of  the  opposition  of  1894,  Mr.  Barnard,  at  the 
Lick  Observatory,  frequently  directed  the  great 
telescope  of  36  inches  of  aperture,  the  instrument 
by  which  he  had  already  discovered  the  fifth  moon 
of  Jupiter,  towards  Mars.  At  times,  when  the 
seeing  was  most  perfect,  although  the  gray-green 
regions  of  the  planet  appeared  richly  covered  by 
delicate  and  intricate  detail,  the  very  suspicion  of 
which  had  never  been  suggested  to  other  observers 
to  whom  the  canals  had  been  so  startlingly  con- 
spicuous, features  were,  indeed,  recognized  upon 
the  orange  continents,  but  they  were  for  the  most 
part  irregular,  and  consisted  only  of  delicate  gra- 
dations of  light  and  shade.  There  was  no  appear- 

(M520)  I 


130          Recent  Advances  in  Astronomy. 

ance  of  hard,  sharp  lines.  A  few  short,  hazy 
streaks  in  the  neighbourhood  of  the  "  Lake  of  the 
Sun  "  appeared  as  nearly  the  sole  representatives  of 
the  Martian  canals. 

To  explain  the  inconsistency  apparent  in  these 
and  other  similar  observations,  it  is  not  for  a 
moment  necessary  to  assume  any  want  of  good 
faith  on  the  part  of  astronomers  to  whom  the 
system  of  the  Martian  canals  has  appeared  in  all 
its  wonderful  complexity.  Experience  has  fully 
shown,  as  eveiy  observer  with  the  telescope  has 
soon  become  painfully  aware,  to  what  a  serious 
extent  the  eye  may  be  deceived  in  its  interpretation 
of  details  so  faint  as  just  to  hover  upon  the  verge 
of  vision,  and  how  readily  unconscious  bias,  the 
result  of  even  faintly  preconceived  ideas,  may  affect 
the  judgment.  Illustrations  are  abundantly  sup- 
plied in  the  history  of  astronomical  observation,  and 
it  will  be  sufficient  to  give  three,  selected  almost 
entirely  at  random.  In  comparing  recent  photo- 
graphs of  the  nebulse  surrounding  the  star  77  Argus 
with  the  beautiful  drawing  of  the  same  object  made 
by  Sir  John  Herschel  during  his  residence  at  the 
Cape,  differences  of  so  startling  a  nature  are  found 
as  to  administer  a  severe  shock  to  those  who  would 
put  their  trust  in  the  eyes  of  man.  The  curiously 
definite  border  assigned,  even  by  so  careful  an 
observer  as  Sir  John  Herschel,  to  a  dark  space  in 
the  nebula,  known  as  the  "  key-hole  ",  when  com- 
pared with  the  perfect  shading  of  light  into  dark- 
ness shown  in  the  photograph,  is  an  indication  of 
the  tendency  of  the  eye  to  assign  to  excessively 


The  Recent  Study  of  Mars.  131 

faint  details  a  sharpness  and  a  regularity  that  they 
do  not  possess.  In  inspecting  sketches  of  the 
delicate  detail  of  the  Corona  of  the  Sun,  made  at 
the  same  time  and  from  the  same  place  by  different 
observers,  it  is  frequently  difficult  to  believe  that 
the  same  object  has  been  represented.  Drawings 
of  the  Milky  Way,  as  seen  by  the  naked  eye,  have 
been  recently  executed  by  two  independent  obser- 
vers, Dr.  Boeddicker  and  M.  Easton,  each  drawing 
the  result  of  long  and  arduous  observation,  but,  in 
comparing  them,  it  is  the  exception  rather  than  the 
rule  to  find  any  approximation  in  agreement  in 
respect  of  the  more  delicate  features. 

Altogether  it  appears  scarcely  possible  to  avoid 
the  conclusion  that  the  existence  of  the  canal  system 
has  not  been  established.  There  is  no  doubt,  how- 
ever, that  in  the  course  of  a  few  years  further  light 
will  be  forthcoming  upon  the  problem.  At  present 
the  planet  is  becoming  at  each  appearance  less 
favourably  situated  for  observation;  but  upon  the 
return  of  favourable  oppositions  in  1907  and  1909, 
its  disc  will  be  scanned  with  an  attention  thoroughly 
aroused  by  the  conflict  of  recent  evidence,  and  with 
the  aid  of  more  powerful  instrumental  means  than 
have  hitherto  been  available.  It  might  also  be  well 
if  each  observer  should,  before  attacking  the  main 
problem,  subject  himself  to  a  severe  examination, 
in  sketching  through  his  telescope  a  number  of 
illuminated  distant  discs,  on  which  faint  markings 
had  been  traced,  but  of  a  nature  unknown  to  him. 
A  personal  tendency  might  be  detected  by  the 
subsequent  comparison  of  the  drawings  with  the 


132          Recent  Advances  in  Astronomy. 

discs,  which  should  serve  as  a  valuable  check  upon 
his  subsequent  observations  of  Mars.  By  the  com- 
parison of  a  number  of  such  carefully  corrected 
records,  it  might  be  confidently  anticipated  that  the 
riddle  of  the  canals  of  Mars  would  receive  its  final 
solution. 

It  has  frequently  appeared  a  grave  difficulty  in 
interpreting  Martian  phenomena  that  the  apparently 
mild  climate,  to  which  they  have  been  generally 
thought  to  point  as  existing  upon  the  planet,  is 
inconsistent  with  its  great  distance  from  the  Sun. 
There  can  be  little  doubt  that  if  the  Earth  were  re- 
moved to  the  distance  of  Mars,  it  would,  by  reason 
of  the  diminished  intensity  of  solar  radiation,  become 
so  much  cooler,  that  nearly  if  not  the  whole  of  its 
oceans  would  be  eternally  bound  in  ice.  Yet  upon 
Mars  the  polar  caps  do  not  extend  to  lower  latitudes 
than  do  those  of  the  Earth,  while  the  polar  ice  on  the 
Earth  is  never  reduced  during  the  hottest  summer 
to  the  insignificant  remnant  by  which  it  is  generally 
represented  in  summer  upon  Mars.  From  these 
facts  it  has  frequently  been  assumed  that  the  climate 
of  Mars  is  actually  warmer  than  that  of  the  Earth. 

Although  it  is  not  possible  to  make  an  exact  esti- 
mate of  the  fall  of  temperature  that  would  result  if 
the  Earth  were  removed  to  the  distance  of  Mars  from 
the  Sun,  a  simple  illustration  will  indicate  its  very 
serious  extent.  Taking  the  approximate  ratio  of  2 
to  3  to  indicate  the  relative  distances  of  the  Earth 
and  Mars  from  the  Sun,  it  follows  that,  since  the 
intensity  of  radiation  is  inversely  proportional  to  the 
square  of  the  distance  of  the  radiating  body,  the 


The  Recent  Study  of  Mars.  133 

heating  effect  of  the  Sun's  rays  at  every  place  upon 
the  Earth's  surface  would  be  reduced  by  the  square 
of  J^,  that  is,  to  4/9  °f  its  present  value.  The  heat- 
ing effect  of  rays  further  depends,  however,  upon 
the  angle  at  which  they  are  incident  upon  the  surface 
that  absorbs  them.  The  greater  the  obliquity,  the 
less  the  heat  developed  upon  equal  areas,  since  the 
more  slanting  the  incidence  the  larger  the  area  over 
which  the  rays  of  a  given  columnar  bundle  would 
be  distributed.  In  passing  from  the  equator  to 
either  pole,  for  instance,  the  heating  effect  of  the 
Sun's  rays  continually  decreases,  as  the  surface 
covered  by  bundles  of  rays  of  equal  section  increases. 
Let  us  now  suppose  the  Earth  to  be  at  an  equinox, 
and  that  it  is  regarded  by  an  observer  stationed  upon 
the  Sun.  Imagine  two  parallel  zones,  each  a  mile 
in  width,  to  be  described  entirely  round  the  Earth 
upon  its  surface,  one  at  the  equator,  and  the  other 
in  latitude  63°.  It  can  be  shown  by  an  exercise  in 
elementary  geometry  that  the  second  is,  by  reason 
of  its  lesser  circumference,  4/9  of  the  first  in  area,  and 
that,  as  seen  from  the  Sun,  it  appears  from  this  cause, 
and  also  from  its  obliquity  to  the  direction  of  vision, 
to  be  (4/9)2  or  l6/8i  as  large  as  the  equatorial  one. 
This  fraction  then  is  the  proportion  between  the 
angles  subtended  at  the  Sun  by  the  zones,  and  it 
therefore  also  represents  that  of  the  quantities  of 
heat  received  by  them  in  equal  times.  The  smaller 
zone  therefore  receives  (4/9)2  of  the  heat  of  the  larger, 
but,  as  its  surface  is  only  4/9  as  great,  the  heat  re- 
ceived by  a  given  area  of  the  smaller  zone  is  4/9  of 
that  received  by  an  equal  area  of  the  larger  one. 


134          Recent  Advances  in  Astronomy. 

But  it  has  been  shown  that,  if  the  distance  of  the 
Earth  from  the  Sun  were  increased  to  that  of  Mars, 
the  heating  effect  of  the  solar  rays  everywhere  upon 
its  surface  would  be  reduced  by  this  amount.  Hence 
the  equatorial  heating  effect  upon  the  Earth,  if  it 
were  transferred  to  the  position  of  Mars,  would  be 
that  at  present  found  in  a  zone  of  63°  latitude.  The 
parallel  of  63°  north  latitude  skirts  the  south  of  Ice- 
land, it  passes  through  Finland,  and  it  traverses 
Northern  Siberia,  Alaska,  the  Hudson  Bay  Terri- 
tory, and  Greenland;  and  there  is  little  doubt  that 
the  temperatures  of  these  regions  are  upon  the  whole 
higher  than  would  result  from  the  direct  radiation 
that  they  receive,  since  the  Arctic  and  adjacent 
regions  are  warmed  by  currents  of  air  from  low^er 
latitudes.  Were  the  Earth,  therefore,  transferred 
to  the  distance  of  Mars,  it  might  be  confidently 
anticipated  that,  even  at  its  equator,  its  climate 
would  be  of  Arctic  character. 

The  phenomena  visible  upon  Mars  do,  however, 
suggest,  though  they  do  not  probably  demonstrate 
as  conclusively  as  has  frequently  been  assumed,  that 
the  temperature  of  the  planet  is  very  different  from 
this,  and  attempts  have  been  made  to  imagine  some 
means  by  which  the  climate  of  the  planet  may  be 
rendered  less  rigorous  than  its  small  allowance  of 
solar  heat  would  suggest.  The  possibility,  one 
indeed  that  has  never  been  seriously  maintained, 
that  the  interior  of  Mars  may  be  hotter  than  the 
Earth,  and  that  its  surface  may  be  appreciably 
warmed  by  the  outward  flow  of  heat  from  its  in- 
terior, may  be  briefly  dismissed.  It  is  probable 


The  Recent  Study  of  Mars.  135 

that  the  interior  of  Mars,  like  the  interior  of  the 
Earth,  is  hotter  than  the  surface ;  but  from  the  pro- 
bability indicated  by  the  nebular  hypothesis,  that 
Mars  was  developed  in  a  highly  heated  condition 
before  rather  than  after  the  Earth,  and  from  the 
certainty  that  its  cooling  must,  by  reason  of  its 
smaller  size,  have  proceeded  far  more  rapidly,  Mars 
should  be  the  colder  of  the  two  planets.  Since,  in 
addition,  measurements  have  shown  that  the  heat 
conducted  from  the  interior  to  the  surface,  even  in 
the  case  of  the  Earth,  is  entirely  insignificant  in 
amount  when  compared  with  that  received  from  the 
Sun,  and  is,  therefore,  a  negligible  quantity  in 
directly  affecting  climate,  it  would  appear  impossible 
that  the  climate  of  Mars  should  be  sensibly  affected 
by  the  internal  heat  of  the  planet. 

The  only  serious  attempt  that  has  been  made  to 
account  for  the  assumed  mild  climate  of  Mars  is 
based  upon  the  property  of  selective  absorption, 
exercised  in  some  degree  by  all,  and  in  a  very 
marked  manner  by  many,  transparent  substances. 
Selective  absorption  is  illustrated  to  a  remarkable 
degree  in  glass.  If  a  plate  of  clear  glass  be  held 
between  the  Sun  and  a  thermometer,  the  bulb  of 
which  should  be  blackened  upon  the  exterior  to 
prevent  the  reflection  of  rays  from  the  metallic  sur- 
face of  the  enclosed  mercury,  it  will  be  found  that 
the  indication  of  the  thermometer  is  scarcely  affected, 
the  glass,  transparent  to  light,  being  similarly  trans- 
parent to  the  greater  part  of  the  rays  that,  upon  their 
incidence  on  the  blackened  surface  of  the  thermo- 
meter, develop  heat.  If,  however,  the  same  glass  is 


136          Recent  Advances  in  Astronomy. 

interposed  in  the  course  of  rays  proceeding  from  a 
red-hot  fire  to  the  thermometer,  a  fall  in  temperature 
will  be  indicated,  almost  as  great  in  amount  as  if  an 
opaque  screen  had  been  substituted  for  the  glass. 
Glass,  therefore,  is  very  transparent  to  the  heat 
radiation  of  the  Sun,  but  is  practically  opaque  to 
that  of  a  red-hot  fire.  It  is  to  this  last  property  that 
the  efficiency  of  a  glass  fire-screen  is  due.  Speaking 
generally,  it  is  found  that  the  higher  the  temperature 
of  a  body  the  more  transparent  is  glass  to  the  heat- 
ing effect  of  its  radiation ;  and  this,  not  from  the 
greater  intensity  of  the  radiation  of  a  hotter  body — 
for  the  experiment  already  described  succeeds  equally 
well  even  if,  as  may  well  be  the  case,  the  glass  is 
placed  so  close  to  the  fire  that  its  radiation  is,  owing 
to  its  close  proximity,  more  intense  than  that  of  the 
Sun  on  its  arrival  at  the  surface  of  the  Earth — but 
by  reason  of  some  property  impressed  upon  the 
radiation  by  the  source.  The  wave  theory  of  light, 
and  of  radiation  in  general,  leaves  little  doubt  that 
the  difference  in  question  is  one  of  rapidity  of  the 
vibration  of  the  ether  of  space  as  it  transmits  the 
waves  that  constitute  radiation,  but  it  is  unnecessary 
here  to  go  beyond  the  actual  property  of  selective 
absorption  as  demonstrated  by  experiment. 

The  familiar  fact  that  upon  a  clear  day  the  air  in- 
side a  greenhouse  may  be  raised  by  the  Sun's  rays 
to  a  temperature  far  in  excess  of  that  outside,  is 
frequently  advanced  as  an  illustration  of  the  effects 
of  selective  absorption,  though  probably  in  this  case 
the  prevention  of  circulation  of  the  enclosed  air  is 
partly  responsible  for  the  rise  in  temperature.  The 


The  Recent  Study  of  Mars.  137 

rays  of  solar  radiation  traversing  the  glass  with 
readiness  arrive  at  and  are  absorbed  by  the  surfaces 
of  the  plants  and  other  objects  exposed  to  them. 
These,  becoming  heated  in  consequence,  radiate 
their  acquired  heat  in  rays  of  essentially  different 
character,  which,  being  effectually  absorbed  by  the 
glass  covering,  cause  it  to  be  heated  by  them. 
Consequently  the  interior,  heated  now  both  by 
radiation  from  the  Sun  and  from  the  warmed  glass, 
acquires  a  temperature  which  is  frequently  far  in 
excess  of  that  which  would  result  if  the  glass  had 
allowed  a  free  path  to  the  radiation  from  the  objects 
within. 

Tyndall  has  shown  that  a  closely  similar  selective 
absorption  may  be  exercised  by  many  gases  and 
vapours*  He  was  unable  to  detect  the  property 
with  certainty  in  dry  air,  but  the  presence  of  a  very 
small  quantity  of  the  vapour  of  water  in  the  air 
produced  it  in  a  marked  degree.  From  the  results 
of  experiments  arranged  with  considerable  care  and 
skill,  he  arrived  at  the  conclusion  that  the  vapour  of 
water  present  in  the  atmosphere  exercises  an  im- 
portant influence  upon  the  meteorology  of  the  Earth, 
permitting  the  transmission  through  the  air  of  the 
solar  rays,  but  largely  arresting  the  heat  upon  its 
return,  by  absorbing  the  radiation  from  the  warmed 
Earth.  It  appeared,  indeed,  that  under  conditions 
in  which  the  atmosphere  contains  an  average  amount 
of  water -vapour  in  England,  as  much  as  10  per 
cent  of  the  Earth's  total  radiation  should  be  arrested 
within  10  feet  of  its  surface. 

Based   upon  the  selective   absorption   of  water- 


138          Recent  Advances  in  Astronomy. 

vapour,  the  interesting  speculation ,  has  been  ad- 
vanced that  a  mild  climate  upon  Mars  may  result 
from  the  distribution  throughout  its  atmosphere  of 
water-vapour,  in  quantity  so  abundant  that,  by  the 
efficiency  of  its  trapping  effect  upon  the  solar  radia- 
tion, it  should  more  than  atone  for  the  great  distance 
of  the  planet  from  the  Sun.  There  is,  however, 
considerable  difficulty  in  imagining  such  a  state  of 
saturation  of  the  Martian  atmosphere  as  probable, 
or  even  possible.  Although  the  physical  conditions 
at  the  surface  of  Mars  do  appear  to  be  in  some  re- 
spects favourable  to  the  formation  of  water-vapour 
in  its  atmosphere,  in  others  they  appear  to  be  ex- 
tremely unfavourable ;  and  though  it  is  not  possible 
to  estimate  accurately  the  opposing  conditions,  there 
can  be  little  doubt  of  the  unfavourable  ones  being 
the  far  more  effective  of  the  two. 

The  conditions  upon  Mars  favourable  to  the 
existence  of  the  vapour  of  water  in  its  atmosphere, 
consist  in  the  low  intensity  of  gravitation  at  the 
surface  of  the  planet,  and  the  probable  tenuity  of 
its  atmosphere.  The  amount  of  water  that  is  ca- 
pable of  existence  in  an  atmosphere  in  the  state  of 
vapour  is  entirely  independent  of  the  density  of 
the  atmosphere;  experiment  showing  that  although 
evaporation  takes  place  more  rapidly  into  rare  air 
than  into  dense,  yet  the  amount  of  vapour  ultimately 
formed  is  the  same,  and  is  still  the  same  even  if 
the  space  into  which  evaporation  takes  place  was 
initially  a  vacuum.  In  every  case  evaporation  pro- 
ceeds until  the  vapour  immediately  in  contact  with 
the  evaporating  surface  has  acquired  a  definite 


The  Recent  Study  of  Mars.  139 

density,  a  density  increasing  with,  and  solely  de- 
pending upon,  the  temperature,  after  which  evapora- 
tion ceases.  In  the  case  of  a  planet,  evaporation 
of  surface-water  would  therefore  continue  until  an 
atmosphere  of  the  vapour  of  water  had  been  formed 
that  should,  independently  of  any  other  atmosphere 
present,  and  therefore  solely  by  its  own  weight, 
produce  such  a  density  in  the  vapour  at  the  surface 
as  would  prevent  further  evaporation.  Since  the 
intensity  of  gravitation  at  the  surface  of  the  Earth 
is  two  and  a  half  times  that  at  the  surface  of  Mars, 
an  extension  of  water  vapour  two  and  a  half  times 
that  necessary  to  prevent  evaporation  at  the  surface 
of  the  Earth  would  be  possible  in  the  atmosphere  of 
Mars. 

In  the  preceding  argument  it  was  necessary  to 
assume  a  very  simple  condition — that  evaporation 
should  steadily  continue  until  the  whole  atmosphere 
had  become  saturated.  It  is  scarcely  necessary  to 
add,  that,  in  the  atmosphere  of  the  Earth,  this  is 
very  far  from  being  the  case.  Owing  to  alterations 
in  temperature  in  extensive  bodies  of  air — due  to 
different  meteorological  changes — condensation  is 
continually  occurring,  resulting  in  the  formation 
of  cloud  and  mist,  and  the  atmosphere  of  the  Earth 
as  a  whole  is,  at  all  times,  very  far  from  being 
saturated.  No  doubt  such  condensation  would  also 
occur  on  Mars,  but  it  is  probable,  from  the  low 
intensity  of  gravitation,  that  the  meteorological 
changes  would  be  less  violent,  and  that  condensa- 
tion would  therefore  be  less  copious.  Also,  under 
conditions  otherwise  similar,  evaporation  would 


140          Recent  Advances  in  Astronomy. 

take  place  more  rapidly  into  the  rarer  atmosphere, 
and  the  loss  of  vapour  due  to  condensation  would 
be  more  quickly  restored.  Perhaps  the  most  de- 
finite form  in  which  it  is  possible  to  express  the 
general  conclusion  is — that  if  the  intensity  of  gravi- 
tation upon  the  surface  of  the  earth  were  reduced, 
and  if,  at  the  same  time,  the  density  of  its  atmos- 
phere were  diminished,  there  is  little  doubt  that 
the  atmosphere  as  a  whole  would  be  more  richly 
charged  with  the  vapour  of  water  than  it  actu- 
ally is. 

In  some  respects,  therefore,  the  physical  condi- 
tions existing  on  Mars  appear  to  be  favourable  to 
the  formation  of  the  vapour  of  water  in  its  atmos- 
phere. On  the  other  hand,  other  conditions  appear 
to  be  so  extremely  unfavourable  that  it  is  difficult 
to  believe  that  these  can  be  of  much  avail.  Evapora- 
tion is  the  direct  result  of  the  radiation  of  the  Sun 
acting  upon  the  surface  of  water.  Not  only  is 
the  intensity  of  the  solar  radiation  upon  Mars  less 
than  one-half  of  its  amount  on  the  surface  of  the 
Earth,  but  the  water  surface  exposed  to  it  appears 
to  be  woefully  restricted.  The  greater  part  of  the 
Earth's  surface  is  occupied  by  water.  A  nearly 
continuous  tropical  belt  of  ocean  is  exposed  day 
after  day  to  the  direct  radiation  of  a  vertical  sun. 
Upon  Mars,  on  the  contrary,  the  tropics  are  al- 
most completely  occupied  by  the  orange  continents. 
Gray-green  regions,  the  aqueous  character  of  which 
is  more  than  doubtful,  extend  over  the  temperate 
zones.  It  is  only  in  the  arctic  regions — the  arctic 
regions  of  a  planet  whose  tropics  receive  heat 


The  Recent  Study  of  Mars.  141 

from  the  Sun  that  compares  unfavourably  in  its 
amount  with  that  received  by  lands  of  ice  and  snow 
upon  the  Earth — that  there  is,  in  the  polar  snows, 
any  indication  of  water  in  either  the  solid  or  the 
liquid  state.  The  faith  of  a  keen  believer  in  the 
habitability  of  Mars  may  see  under  such  conditions 
an  atmosphere  heavily  laden  with  moisture,  but  to 
us  it  appears  that  poor  success  has  accompanied 
the  attempt  to  warm  Mars  by  a  cloak  of  vapour. 

Further,  it  appears  certain  that  the  trapping  effect 
of  the  vapour  of  water  has  been  much  overestimated. 
If  we  accept  Tyndall's  estimate,  that  under  average 
conditions  in  this  country,  10  per  cent  of  the  heat 
radiated  by  the  Earth  is  absorbed  within  10  feet  of 
its  surface,  it  follows,  as  has  been  pointed  out  by 
Lord  Kelvin,  that  so  high  a  rate  of  absorption  can- 
not continue;  for  if  it  did,  10  per  cent  of  the  heat 
escaping  absorption  in  the  first  10  feet  being  ab- 
sorbed in  the  next  10,  and  so  on,  90  per  cent,  or 
nearly  the  whole,  would  be  absorbed  in  200  feet,  a 
conclusion  that  is  directly  contradicted  by  the  very 
marked  effect  of  cloud  in  checking  the  fall  of  tem- 
perature by  radiation  from  the  Earth's  surface.  It 
is  probable  that  water  vapour  absorbs  only  a  few 
waves  of  definite  lengths  among  the  many  compos- 
ing terrestrial  radiation,  that  the  very  rapid  absorp- 
tion of  these  gives  rise  to  the  strongly-marked  effect 
actually  observed,  but  that  the  remaining  waves, 
bereft  of  their  more  susceptible  companions,  escape 
without  much  further  loss. 

To  reconcile  the  dissipation  of  the  polar  caps  with 
the  intense  cold  that  it  appears  necessary  to  regard 


142          Recent  Advances  in  Astronomy. 

as  prevailing  over  the  Martian  world,  it  has  been 
suggested  by  Mr.  Cowper  Ranyard  and  other 
astronomers  that  the  Martian  snows  may  be  the 
solidified  form  of  some  liquid  other  than  water,  and 
freezing  at  a  lower  temperature.  The  occurrence  of 
carbonic  acid  gas  as  a  constituent,  howbeit  a  minor 
one,  of  the  Earth's  atmosphere,  and  the  fact  that 
by  extreme  cold  it  becomes  condensed  as  a  white 
powder,  very  closely  resembling  snow  in  appear- 
ance and  melting  at  a  temperature  of  about  120 
Fahrenheit  degrees  below  the  freezing-point  of 
water,  has  pointed  to  it  as  the  origin  of  the  polar 
caps  on  Mars.  There  are,  however,  very  serious 
objections  to  this  view.  Under  ordinary  conditions 
of  pressure,  carbonic  acid  is  incapable  of  assuming 
the  liquid  state,  the  solid  upon  being  heated  passing 
directly  into  the  condition  of  gas.  Under  consider- 
able pressure,  however,  the  heated  solid  does  melt, 
the  resulting  liquid  boiling  at  a  still  higher  tem- 
perature and  becoming  a  gas.  The  least  pressure 
necessary  for  this  purpose  is  about  five  times  that 
of  the  atmosphere  at  the  surface  of  the  Earth. 
Hence  for  liquid  carbonic  acid  to  exist  on  Mars, 
in  consequence  of  the  low  intensity  of  gravitation, 
12^2  times  the  mass  of  air  must  be  accumulated 
over  each  square  mile  as  is  accumulated  over  a 
square  mile  of  the  Earth,  an  estimate  that  cannot 
possibly  be  accepted.  If,  therefore,  the  polar  snows 
consist  of  solid  carbonic  acid,  they  must  be  formed 
by  a  direct  precipitation  of  the  hoar-frost  of  carbonic 
acid,  and  their  disappearance  must  be  a  similarly 
direct  process  of  evaporation.  This  conclusion  is 


The  Recent  Study  of  Mars.  143 

directly  challenged  by  the  appearance  to  Mr.  Lowell 
and  Professor  Pickering  of  the  blue-black  belt  fring- 
ing the  disappearing  cap  of  1894,  and  the  evidence 
that  it  furnished  as  to  its  liquid  nature. 

The  actual  deposition  and  dissipation  of  the  hoar- 
frost of  water  is  not  inconsistent  with  a  temperature 
considerably  below  the  freezing-point,  since  direct 
evaporation  takes  place  from  ice  at  such  low  tem- 
peratures. Were  it  not  for  the  evidence  of  the 
fringing  belt,  the  gray-green  regions  might  well  be 
ice-bound  seas,  from  which  evaporation  would  take 
place  under  the  cloudless  Martian  skies.  The  air 
would  thus  become  charged  to  a  slight  extent  with 
the  vapour  of  water,  which,  distributed  over  the 
planet  by  atmospheric  circulation,  would  be  ulti- 
mately deposited  as  frost  on  the  coldest  polar 
regions. 

In  common  with  the  greater  number  of  other 
celestial  objects  to  which  it  has  become  possible  to 
apply  a  detailed  examination,  Mars  has  passed 
through  a  first  stage  in  which  it  appeared  a  simple 
and  an  easy  thing  to  interpret  the  features  revealed, 
and  has  reached  another,  in  which  the  first  pleasing 
views  have  been  rudely  shaken,  as  observation  has 
revealed  difficulties  at  a  far  greater  rate  than  it  has 
solved  them.  Could  we  but  traverse  the  millions 
of  miles  of  planetary  space  that  separate  us  from 
our  ruddy  neighbour,  and  dwell  for  a  time  upon 
its  surface,  we  should  look  around  us  in  vain  for 
evidence  of  that  fair  miniature  of  the  world  we  had 
left  that  formed  the  romantic  picture  of  our  fathers. 
It  is  more  likely  that  we  should  find  in  Mars  a 


144          Recent  Advances  in  Astronomy. 

succession  of  bleak  arid  deserts  over  which  the  rays 
of  the  vertical  Sun  would  seem  to  struggle  in  vain 
to  mitigate  the  blasting  chill  of  the  attenuated  air. 
We  should  find,  in  higher  latitudes,  a  succession 
of  plains,  clothed,  perhaps,  with  elementary  forms 
of  vegetation  capable  of  withstanding  the  rigours 
of  a  climate  more  than  arctic  in  character.  We 
should  possibly  encounter  animal  life,  but  assuredly 
in  no  familiar  form.  With  the  whole  aspect  of 
nature  it  would  be  difficult  to  associate  romance, 
and  we  should  be  well  content  for  the  future  to  limit 
our  acquaintance  with  the  planet  to  the  softened 
picture  presented  in  the  field  of  view  of  a  telescope 
mounted  on  the  more  genial  Earth. 


Chapter  IV. 
The  Analysis  of  Sunlight. 

In  the  year  1672  Sir  Isaac  Newton  published, 
among  other  discoveries  in  optics,  the  account  of 
an  experiment,  in  principle  closely  agreeing  with 
one  less  perfectly  arranged  and  interpreted  by 
John  Kepler  more  than  half  a  century  before,  that 
was  to  form  the  foundation  of  a  new  branch  of 
physics;  one  that,  in  its  application  to  Astronomy 
a  century  and  a  half  later,  was  destined  to  renew 
the  youth  of  the  oldest  of  the  sciences,  not  unfre- 
quently  regarded  then  as  approaching  the  termina- 
tion of  its  active  career  and  as  having  achieved  its 
last  triumphs.  Newton's  experiment  was  that  of 


The  Analysis  of  Sunlight.  145 

the  analysis  of  sunlight,  and  the  science  that  owes 
its  origin  to  it  is  Spectrum  Analysis. 

In  Newton's  experiment,  a  circular  hole  was 
bored  in  the  shutter  of  an  otherwise  darkened  room, 
and  through  it,  when  the  Sun  was  unclouded  in 
the  sky  beyond,  a  beam  of  sunlight  penetrated  the 
room,  and,  following  a  straight  course,  formed  an 
oval  spot  of  white  light  upon  the  opposite  wall.  A 
prism — a  block  of  triangular  section — of  glass  was 
then  placed  in  the  path  of  the  beam  and  immediately 
against  the  shutter,  and,  as  the  result,  the  white  spot 
disappeared,  and  was  replaced  by  a  luminous  band 
of  coloured  light  considerably  displaced  from  it  in 
position.  The  band  displayed  from  one  end  to  the 
other  a  series  of  colours  which  closely  corresponded 
with  those  seen  in  the  rainbow,  red  appearing 
nearest  the  original  position  of  the  white  spot,  and 
violet  at  the  end  farthest  from  it. 

From  the  fact  of  the  displacement  of  the  luminous 
image  on  the  wall,  the  prism  clearly  exercised  a 
deflecting  effect  upon  the  beam  of  light;  and  New- 
ton accounted  for  the  appearance  of  colour  by  the 
supposition  that  the  light  of  the  Sun  was  compound 
in  nature,  being,  in  fact,  a  mixture  of  all  colours  of 
the  rainbow ;  that  the  combined  effect  of  the  whole 
upon  the  eye  was  to  develop  the  sensation  of  white ; 
but  that  the  differently-coloured  rays  were  deflected 
in  different  degrees  in  traversing  the  prism;  red 
experiencing  the  least,  violet  the  greatest,  deflection, 
while  the  colours  appearing  between  these  were 
deflected  to  intermediate  and  different  extents.  The 
complete  series  he  described  as  red,  orange,  yellow, 

(M520)  K 


146 


Recent  Advances  in  Astronomy. 


green,  blue,  indigo,  and  violet;  bu,t  it  is  probable 
that  to  most  eyes  the  blue  and  indigo  would  appear 
as  only  different  shades  of  the  same  colour.  New- 
ton supported  his  explanation  by  a  number  of  simple 
though  ingeniously-arranged  experiments,  and  no 
doubt  has  since  existed  as  to  its  soundness. 

Fig.  7  represents,  in  simple  diagrammatic  form, 


Fig.  7. — Newton's  Experiment 

a  vertical  section  of  the  arrangement  of  Newton's 
experiment,  and  requires  but  little  explanation,  s 
is  the  section  of  the  shutter,  A  that  of  the  circular 
hole.  The  Sun  being  in  the  direction  of  AC,  a 
beam — that  is,  a  bundle  of  its  rays — arrives  from 
along  CA,  enters  the  hole,  and  in  the  absence  of  the 
prism  would  traverse  the  room  and  form  a  white 
spot  upon  the  opposite  wall  at  B.  Upon  interpos- 
ing the  prism,  however,  at  P,  the  rays  suffer  deflec- 


The  Analysis  of  Sunlight.  147 

tion,  and  are  thrown  upward,  forming  a  coloured 
band  between  the  limits  R  and  v,  the  least  deviated 
— the  red  rays — arriving  at  R,  and  the  most  deviated 
— the  violet — at  v,  while  the  original  white  spot 
disappears. 

The  reader,  if  generally  unacquainted  with  the 
first  principles  of  optics,  will  probably  find  it  well 
to  trace  the  action  of  the  prism  on  light  in  the  more 
thorough  manner  developed  in  the  next  few  para- 
graphs, but,  should  the  above  explanation  appear 
entirely  satisfactory  and  complete,  these  may  be 
passed  over. 

It  is  a  matter  of  common  experience  that  light 
behaves  normally — at  any  rate,  to  a  very  close 
degree  of  approximation — as  if  it  travelled  along 
straight  lines.  Were  it  not  so,  a  distant  object 
would  not  become  hidden  by  the  interposition  of  an 
opaque  screen  in  the  straight  line  between  it  and 
the  eye.  As  a  matter  of  fact,  refined  observations 
of  the  phenomena  included  under  the  term  "diffrac- 
tion" indicate  that  the  transmission  of  light  is  not 
completely  expressed  in  so  simple  a  statement,  but 
no  error  will  arise  from  its  assumption  in  the  pres- 
ent instance.  To  light  thus  regarded  as  proceeding 
along  a  straight  line,  the  term  "  ray"  is  applied. 

A  ray  of  light  continues  its  path  in  a  straight  line, 
however,  only  for  so  long  as  the  substance — or 
medium — through  which  it  is  transmitted  is  of 
absolute  uniformity.  It  is  a  matter  of  common 
knowledge  that,  in  passing  from  one  medium  into 
another,  the  course  of  light  is  deflected  or  "  re- 
fracted" at  the  separating  surface.  Thus,  in  fig.  8, 


148 


Recent  Advances  in  Astronomy. 


let  PQ  indicate  the  course  of  a  ray  in  air  incident 
upon  the  surface  of  water  at  Q.  The  path  of  the 
ray  within  the  water  will  still  be  along  a  straight 
line  QR,  but  this  will  not  be  the  continuation  of  its 
former  direction.  It  has  been  found  that,  in  travel- 
ling from  a  rarer  into  a  denser  medium,  a  ray  of 
light  is  commonly  refracted  towards  the  normal — or 

perpendicular  —  to 
,/T  the  separating  sur- 

face  at  the  point 
of  incidence;  and 
that,  conversely,  in 
its  passage  from  a 
denser  into  a  rarer 
one,  it  is  deflected 
from  the  normal. 
Thus,  the  ray  PQ, 
if  continued  in  its 
original  direction, 
would  proceed 
along  QS;  but,  on 
entering  the  water, 

it  is  actually  deflected  towards  the  normal  NN'  into  the 
direction  QR.  In  travelling  the  reverse  way,  a  ray, 
following  the  course  RQ  while  within  water,  would, 
upon  entering  air,  be  deflected  from  the  normal  into 
the  direction  QP.  Only  a  ray  incident  normally  upon 
a  separating  surface  penetrates  it  without  experienc- 
ing refraction.  From  the  refraction  of  light  follows 
the  familiar  fact  that  an  object  in  one  medium,  when 
viewed  from  another,  generally  appears  to  be  in  a 
direction  different  from  that  in  which  it  really  is 


Fig.  8.— Refraction  of  Light. 


The  Analysis  of  Sunlight. 


149 


Thus,  once  more  referring  to  the  figure,  one  of  the 
rays  proceeding  from  an  object  situated  at  R  would 
follow  the  paths  RQ  and  QP  in  succession,  and, 
should  it  enter  the  eye  at  P,  the  object  will  appear 
as  if  it  were  in  the  direction  PS,  since  it  is  from  this 
direction  that  the  ray  arrives.  By  analogous  reason- 
ing, an  object  at  P,  as  seen  by  an  eye  situated  below 
the  surface  of  water  at  R,  would  seem  to  be  in  the 
direction  RT. 

The  amount  of  deflection  experienced  by  a  ray  in 
the  act  of  refraction  depends  upon  the  angle  at 
which  it  is  incident  upon  the  refracting  surface,  as 
well  as  upon  the  natures  of  the  two  media.  The 
exact  law  by  which  it  is  determined  was  discovered 
by  Snell  about  the  year  1621,  but  its  statement  is 
unnecessary  for  the  purpose  of  the  present  study. 
The  fact  of  re- 
fraction, as  well 
as  Snell's  law, 
are  simply  ex- 
plained by  the 
wave  theory  of 
light. 

In  fig.  9 
the  paths  are 
traced  of  three 
rays  traversing 
a  transparent 

prism  of  glass,  the  amount  of  refraction  being  in 
each  case  calculated  from  Snell's  law.  The  manner 
in  which  the  rays  are  always  deflected  towards  the 
normal  upon  entering  the  glass,  and  from  the  nor- 


Fig.  9. — Paths  of  Rays  traced  through  a 
Glass  Prism. 


150          Recent  Advances  in  Astronomy. 

mal  on  leaving  it,  should  be  carefully  followed, 
and  the  construction  of  similar  diagrams  for  other 
prisms  of  different  vertical  angles  would  form  an 
instructive  exercise.  It  is  found  that  in  every  case 
the  ray,  by  its  passage  through  a  prism,  is  deflected 
from  the  refracting  edge,  or  that  at  which  the  two 
faces  concerned  in  the  refraction  meet. 

Experimenting  with  lights  of  different  colours,  it 
is  found  that  they  experience  refraction  in  different 
degrees,  red  experiencing  the  least  and  violet  the 
greatest  deviation.  In  this  lies  the  complete  ex- 
planation of  Newton's  experiment.  A  very  great 
number  of  colours  and  shades  of  colour  are  present 
in  the  light  of  the  Sun.  All  of  them  coexist 
throughout  the  whole  of  the  beam  that  enters  the 
room.  On  traversing  the  prism,  the  violet  rays  in 
the  beam  are  deflected  far  from  the  refracting  edge, 
and  by  themselves  would  form  a  violet  spot  on  the 
wall,  largely  displaced  from  the  position  of  the 
original  white  one.  The  red  rays  would  by  them- 
selves form  a  red  spot,  displaced  to  a  less  extent, 
while  each  colour  and  shade  of  colour  would  form  a 
corresponding  spot  of  coloured  light  between  these 
two  extremes.  The  coloured  band — or  spectrum — 
is  therefore  formed  by  a  number  of  coloured  spots, 
one  formed  by  each  colour  and  shade  of  colour 
present  in  the  original  light. 

This  explanation  of  the  formation  of  the  spectrum, 
although  beyond  doubt  the  correct  one,  is  not  free 
from  difficulty,  for  it  is  truly  wonderful  that  the 
very  definite  and  distinct  sensations  of  colour  that 
are  produced  by  the  rays  separately  should  so 


The  Analysis  of  Sunlight.  151 

entirely  disappear  in  the  white  that  results  from  the 
excitement  of  all  of  them  simultaneously.  One  of 
the  most  direct  evidences  of  the  soundness  of  the 
theory  is  that  when,  as  may  be  effected  by  several 
simple  methods,  the  colours  of  the  spectrum  are 
recombined,  a  perfect  white,  indistinguishable  from 
the  original,  appears  as  the  result  of  the  mixture. 

The  number  of  colours  represented  in  the  spec- 
trum of  sunlight,  as  apparent  to  a  normal  eye,  is 
generally  regarded  as  seven,  though  it  is  probable 
that  most  observers  would  suggest  six.  Since  the 
different  colours  owe  their  appearance  to  their  being 
refrangible  in  different  degrees,  the  experiment  may 
at  first  suggest  the  view  that  there  are  in  sunlight 
seven,  and  only  seven,  different  kinds  of  light,  each 
of  a  definite  refrangibility.  If  this  were  the  case, 
the  spectrum  would  be  formed  of  seven  coloured 
patches.  If  the  hole  in  the  shutter  were  large,  the 
section  of  the  beam,  and  consequently  each  coloured 
patch,  would  be  correspondingly  large:  and  ad- 
jacent, or  even  non-adjacent  patches,  might  overlap ; 
but,  by  making  the  hole  small,  and  so  restricting 
the  section  of  the  beam,  it  should  be  possible  to  get 
rid  of  this  confusion,  since  the  angular  separation 
effected  by  the  prism  would  remain  unchanged, 
while  each  coloured  patch  would  decrease  in  size. 
Thus,  let  the  row  of  seven  black  dots  repeated  in 
the  first  three  lines  of  fig.  10  represent  by  their 
positions  the  amount  of  separation  of  the  supposed 
seven  different  kinds  of  light  in  sunlight.  Then, 
with  a  very  small  hole  in  the  shutter,  seven  small, 
separate,  and  differently-coloured  spots  would  ap- 


152 


Recent  Advances  in  Astronomy. 


pear,  as  indicated  in  the  first  line ;  while,  by  increas- 
ing the  aperture,  the  size  of  the  spots  would 
increase  and  overlapping  would  occur,  as  repre- 
sented in  the  second  and  third  lines,  a  spectrum 
being  formed,  in  which,  as  in  the  actual  spectrum 

/a^^/a^^^  of  sunli§"ht>  each  co1- 

our  would   pass   into 

the  next  by  insensible 
gradations. 

That  the  perfect 
gradation  of  tint  in 
the  spectrum  of  sun- 
light is  not  due  to  the 
overlapping  of  only 
seven  differently-col- 
oured patches,  is,  how- 
ever, shown  by  the 
fact  that  it  is  quite  im- 
possible to  reduce  the 
aperture  to  such  dim- 
ensions that  the  spec- 
trum shall  be  resolved  into  separate  patches  of 
colour.  However  small  the  aperture,  and  however 
distant  the  screen,  the  removal  of  which  to  a  greater 
distance  would  have  the  same  effect  in  tending  to 
separate  the  coloured  patches  as  reducing  the 
aperture,  the  different  patches  pass  the  one  into  the 
other  by  perfectly  insensible  gradations.  It  is 
true  that,  with  sunlight,  breaks  in  the  band  might 
ultimately  appear,  due,  as  will  be  seen  later,  to 
another  cause;  but  by  substituting  the  light  of  a 
candle-  or  lamp-flame  for  that  of  the  Sun,  the 


Fig.  10. — Formation  of  Pure  and 
Impure  Spectra. 


The  Analysis  of  Sunlight.  153 

spectrum  would  continue  unbroken  from  end  to 
end. 

The  impossibility  of  separating  the  coloured 
patches  indicates  that  there  are,  for  all  practical 
purposes,  an  infinite  number  of  colours  in  the  light 
of  the  Sun  or  that  of  a  candle.  The  description  of 
the  spectrum  as  consisting  of  seven  colours  indi- 
cates that  in  the  gradual  transition  through  the 
infinite  series  of  rays  from  one  extreme  of  refrangi- 
bility  to  the  other,  seven  fundamentally  different 
sensations  are  successively  excited.  The  seven 
colours  of  the  spectrum  have  reference  to  a  physio- 
logical, not  to  a  physical  fact. 

Instead  of  allowing  the  rays,  after  their  separation 
by  the  prism,  to  illuminate  a  wall  or  other  screen 
before  being  appreciated  by  the  eye,  they  may  be 
received  by  the  eye  directly;  and,  with  most 
sources  of  light,  this  is  the  only  means  by  which  a 
sufficiently  brilliant  result  can  be  obtained.  The 
optical  principles  involved  in  this  method  of  view- 
ing the  spectrum  will  be  clear  from  the  construction 
of  fig.  n.  Here  A  represents  a  small  hole  in  a 
screen,  supposed  to  be  placed  in  front  of  a  candle- 
flame,  and  E  the  eye  of  an  observer,  which  should, 
however,  be  placed  in  practice  immediately  behind 
the  prism.  A  ray  of  red  light  traversing  a  definite 
point  in  the  hole  will  follow  some  such  course  as 
ABE,  and  will  enter  the  eye.  As  the  result,  the  eye 
will  picture  the  position  of  A  at  some  point,  such  as 
R,  in  the  direction  from  which  the  ray  arrived. 
The  red  rays  traversing  all  points  of  the  hole  will 
behave  in  a  similar  manner,  and  the  whole  collec- 


154          Recent  Advances  in  Astronomy. 

tion  will  appear  as  originating  from  a  red  circle  at 
R.  Such  a  reproduction  of  the  appearance  of  the 
hole,  a  ghost  from  which  the  rays  appear  to  come, 
is  technically  called  its  image.  A  violet  ray 
traversing  the  same  point  in  the  hole,  and  accom- 


Fig.  n. — The  Principle  of  the  Spectroscope. 

panying  the  original  red  ray  along  AB,  will 
experience,  in  traversing  the  prism,  a  greater 
deflection  than  the  red  ray,  and  will  be  thrown  into 
the  direction  ABC.  It  will,  therefore,  miss  the 
eye  altogether,  and  will  be  ineffective;  but  some 
other  violet  ray,  such  as  AD,  starting  in  a  direction 
still  more  removed  than  the  red  ray  from  the  direc- 
tion of  the  eye,  will,  by  its  greater  refraction  by  the 
prism,  enter  the  eye,  and  will  appear  as  if  it  came 
from  v.  The  red  rays,  therefore,  will  develop  in 
the  eye  an  appearance  of  a  red  image  of  the  hole  at 
R,  and  the  violet  rays  will  similarly  develop  the 


The  Analysis  of  Sunlight.  155 

appearance  of  a  violet  image  of  the  hole  at  v. 
Every  other  colour  present  in  the  light  will  similarly 
develop  a  corresponding  image  of  the  hole  in  its 
own  colour,  in  position  intermediate  between  R  and 
v,  and  the  entire  series,  which  will  of  course  over- 
lap, as  when  projected  upon  a  screen,  will  form  the 
spectrum  of  the  light  under  examination.  If  the 
screen  in  front  of  the  candle  were  removed,  the  red 
rays  emitted  by  every  point  of  the  flame  would  give 
rise  to  the  appearance  of  a  red  flame  at  R;  the 
violet  rays  to  that  of  a  violet  flame  at  v ;  and  the 
other  colours  behaving  similarly,  there  would 
result  the  appearance  of  a  very  impure  spectrum 
formed  by  a  great  number  of  overlapping  pictures 
of  the  flame  in  all  the  colours  present  in  the  light. 

In  1802,  Wollaston  effected  a  great  improvement 
in  the  method  of  obtaining  the  spectrum,  by  trans- 
mitting the  light  to  be  examined  through  a  fine  slit 
instead  of  through  a  round  hole.  The  slit  was  ar- 
ranged parallel  to  the  edge  of  the  prism.  By  this 
arrangement,  the  overlapping  of  images  was  very 
much  reduced,  or,  in  technical  language,  the  purity 
of  the  spectrum  was  very  much  increased.  Every 
colour  present  in  the  light  would  now  be  represented 
in  the  spectrum  by  a  narrow  band,  which  would 
become  a  fine  line  if  the  slit  were  sufficiently 
narrow;  and  these  would  overlap  to  a  far  less 
extent  than  the  images  of  a  round  hole  that  should 
allow  of  the  passage  of  an  equal  amount  of  light. 
If,  for  instance,  under  the  conditions  assumed  in 
fig.  10,  a  slit  were  substituted  for  the  hole,  the 
appearance  would  be  as  represented  in  the  fourth 


156          Recent  Advances  in  Astronomy. 

line,  and  the  confusion  occurring  in  the  two  im- 
mediately above  it  would  be  entirely  avoided.  It 
is  of  the  highest  importance  to  continually  bear  in 
mind  that  the  appearance  of  the  spectrum,  whether 
formed  by  projection  upon  a  screen  or  viewed 
directly  by  the  eye,  is  due  to  a  series  of  pictures 
or  images  of  the  aperture  by  which  the  light  is 
admitted ;  a  separate  image  being  formed  by  each 
colour,  or,  more  definitely,  by  each  kind  of  light, 
as  determined  by  its  degree  of  refrangibility,  repre- 
sented in  the  original  light. 

Admitting  sunlight  through  such  a  fine  slit,  and 
viewing  the  slit  through  a  glass  prism,  Wollaston 
perceived  the  spectrum  to  be  crossed  at  right  angles 
to  its  length  by  four  diffused  dark  lines.  The  lines 
were  not  seen  in  the  spectrum  of  a  candle-flame  or 
with  other  artificial  sources  of  illumination.  A 
first  glimpse  was  thus  obtained  of  a  discovery  that 
was  in  a  few  years  to  revolutionize  the  study  of  the 
Sun  and  stars. 

In  1814  further  refinements  were  introduced  by 
the  celebrated  instrument-maker,  Fraunhofer  of 
Munich.  Fraunhofer's  remarkable  skill  as  an 
optician  enabled  him  to  construct  prisms  of  finer 
quality  and  with  faces  more  truly  plane  than 
had  been  found  possible  before,  both  points  of 
great  importance  in  giving  accurate  definition  to 
the  images  produced ;  while,  instead  of  viewing  the 
spectrum  directly,  it  was  examined  through  a  tele- 
scope, which  received  the  rays  immediately  after 
their  passage  through  the  prism.  As  the  result, 
the  dark  lines  faintly  seen  by  Wollaston  became 


The  Analysis  of  Sunlight.  157 

far  more  distinct,  and  their  number  was  increased 
to  576.  Some  of  them  were  also  recognized  in  the 
spectra  of  planets  and  of  fixed  stars. 

A  further  refinement,  and  one  by  which  the  pris- 
matic spectroscope  practically  acquired  its  present 
form,  was  effected  by  Simms,  another  famous 
instrument-maker,  in  1839.  The  image  formed  by  a 
prism,  or  by  the  refraction  of  light  at  a  single  plane 
surface,  is  not,  excepting  under  special  conditions, 
sharply  defined.  This  is  due  to  the  fact  that,  even 
with  light  of  a  pure  colour,  and,  therefore,  of  only 
one  degree  of  refrangibility,  rays  originating — and 
therefore  diverging — from  a  point,  do  not  after 
refraction  diverge  from  any  one  point,  a  defect  that 
arises  from  the  particular  form  of  the  law  of  refrac- 
tion and  that  we  shall  make  no  attempt  to  explain 
here.  Since,  owing  to  its  dimensions,  the  eye 
receives  not  one  ray  of  light  but  a  number  of  rays ; 
and  since  the  refracted  rays  do  not  diverge  from  a 
point,  the  image  appears  slightly  out  of  focus  and 
indistinct.  Light  is  diffused  beyond  what  should 
otherwise  be  the  sharp  limits  of  the  image,  and 
overlapping  of  neighbouring  images  in  the  spectrum 
is  unduly  pronounced.  If,  however,  the  object  is 
so  far  distant  from  the  prism  that  the  rays  falling 
upon  the  face  of  the  prism  are  sensibly  parallel,  all 
are  refracted  to  the  same  extent,  the  emergent  rays 
are  still  parallel,  and  indistinctness  of  the  image 
due  to  this  cause  does  not  occur.  Fraunhofer,  who 
was  the  first  to  appreciate  the  importance  of  this 
condition,  had  been  careful  to  approximate  as 
close  as  possible  to  it,  by  placing  the  prism  at  a 


158          Recent  Advances  in  Astronomy. 

considerable  distance — in  some  cases  as  much  as 
24  feet — from  the  slit;  but  the  method  was  in- 
convenient, and  involved  a  waste  of  light;  since 
the  greater  number  of  the  rays  diverging  from  the 
slit  missed  the  prism  altogether.  The  improve- 
ment effected  by  Simms  consisted  in  introducing  a 
condensing  lens  in  the  path  of  the  rays  between  the 
slit  and  the  prism,  at  a  distance  equal  to  its  focal 
length  from  the  slit.  The  lens,  known  as  the 
"collimating  lens",  collected  the  conical  bundle  of 
rays  diverging  from  any  point  of  the  slit,  and  con- 
densed them  into  a  sheaf  of  parallel  rays  before 
they  fell  on  to  the  nearest  face  of  the  prism.  With 
a  collimating  lens,  the  slit  need  not,  therefore,  be 
farther  from  the  prism  than  the  focal  length  of  the 
lens,  which  may  be  only  a  few  inches.  By  these 
means  distortion  of  the  image  due  to  the  divergence 
of  the  incident  rays  was  entirely  obviated. 

Reference  must  be  made  to  one  other  condition 
of  spectral  purity,  familiar  to  Newton  and  to  later 
workers  upon  the  analysis  of  light.  It  can  be 
shown  to  follow  from  the  form  of  the  law  of  refrac- 
tion, that,  other  conditions  being  the  same,  the 
definition  of  the  image  is  least  imperfect  when  the 
path  of  the  ray  in  the  prism  makes  equal  angles 
with  the  refracting  faces,  a  case  illustrated  by 
the  central  of  the  three  rays  traced  in  fig.  9.  In 
practical  work  with  the  spectroscope,  the  prism 
is  always  adjusted  for  this  to  be,  as  nearly  as 
possible,  the  case.  The  adjustment  is  made  by 
turning  the  prism  to  and  fro  until  it  is  observed 
that  the  spectral  image  is  displaced  from  the  direc- 


The  Analysis  of  Sunlight.  159 

tion  of  the  slit  to  the  least  possible  extent;  optical 
theory  having  shown  that  this  condition  of  "  mini- 
mum deviation"  is  coincident  with  the  desired 
symmetrical  passage  of  the  ray  through  the  prism. 
There  should  now  be  little  difficulty  in  following 
the  theory  of  the  modern  prismatic  spectroscope, 
a  diagrammatic  section  of  which  is  given  in  fig.  12. 


Fig.  12. — The  Spectroscope.1 

The  slit  is  formed  by  bringing  the  carefully  finished 
edges  of  two  metal  plates  a  a,  almost  into  contact. 
By  an  adjusting  screw,  one  of  these  can  be  moved 
towards  or  away  from  the  other,  by  which  means 
the  slit,  which  is  to  be  regarded  as  standing  perpen- 
dicularly to  the  paper  at  A,  can  be  made  wide  or 
narrow.  The  rays  of  light  that  enter  the  instrument 
by  any  point  of  the  slit  travel  down  a  metal 
u  coll i mating  tube"  B,  usually  from  i  to  3  feet  in 
length,  at  the  end  of  which  they  fall  upon  the  colli- 
mating  lens  c,  by  which  their  paths  are  rendered 
parallel.  Since  everyone  of  them  falls  upon  the 
first  surface  of  the  prism  at  the  same  angle,  all 

1  The  telescope  and  the  collimator  are  usually  from  two  to  three  times 
longer  in  comparison  with  their  diameters  than  is  represented  in  the  figure, 
in  which  their  sectional  dimensions  are  exaggerated  for  the  sake  of  clear- 


160          Recent  Advances  in  Astronomy. 

of  the  same  colour  are  refracted  equally,  and  their 
courses  on  traversing  the  prism,  and  after  leaving 
it,  are  still  parallel.  Following  now  rays  of  one 
colour  only,  represented  by  the  unbroken  lines  in 
the  figure,  they  pass  on  to  the  telescope.  The 
object-glass  o,  a  condensing  lens  similar  to  that 
of  the  collimator,  condenses  then  to  a  focus  at  v. 
On  passing  this  they  diverge  again,  traverse  the 
eye-piece,  and  are  received  by  the  eye,  to  which, 
as  the  result,  the  appearance  is  presented  of  a  line 
of  light — an  image  of  the  slit — of  a  colour  defined 
by  the  nature  of  the  rays,  and  in  position  by  the 
degree  to  which  their  courses  have  undergone  de- 
flection by  the  prism.  Regarding,  for  instance,  the 
rays,  the  paths  of  which  have  been  traced,  as  violet; 
the  red,  represented  by  the  dotted  lines  in  the  figure, 
experiencing  less  deflection,  would  be  focussed  in 
some  such  position  as  R,  and  other  rays  would  be 
condensed  in  positions  intermediate  between  R  and 
v.  As  in  the  previous  cases,  a  coloured  line  appears 
in  the  field  of  view  for  each  kind  of  light  radiated 
by  the  source.  When  the  light  has  been  sufficiently 
powerful  to  admit  of  a  greater  extension  of  its  spec- 
trum without  undue  enfeeblement  of  its  colours,  two 
or  more  prisms  have  sometimes  been  inserted  be- 
tween the  collimator  and  telescope  in  such  a  manner 
that  the  light  traverses  all  of  them  in  succession. 
The  separation  effected  is,  of  course,  in  direct  pro- 
portion to  the  number  of  prisms  employed. 

Recently  a  "  diffraction  grating"  —  a  surface  of 
polished  silver  ruled  very  closely  by  a  diamond 
with  a  number  of  parallel  lines — has  been  frequently 


The  Analysis  of  Sunlight.  161 

substituted  for  the  prism.  The  light  from  the  colli- 
mator  falls  upon  the  grating,  and  is  thrown  back, 
separated  in  the  act  of  reflection  into  its  constituent 
colours.  A  spectrum  is  thus  formed,  which  is 
examined  through  a  telescope,  arranged  nearly 
parallel  to  the  collimator,  and  by  the  side  of  it.  It 
is  not  possible  to  give  here  the  explanation  of  the 
analysis  of  light  by  the  diffraction  grating,  which 
is,  however,  in  perfect  accord  with  the  wave  theory 
of  light.  The  separation  of  the  colours  is  propor- 
tional to  the  closeness  of  the  lines,  and  gratings 
recently  constructed  by  Professor  Rowland  of 
Baltimore  contain  as  many  as  40,000  lines  to  the 
inch.  The  diffraction  spectrum  possesses  certain 
advantages  and  disadvantages  as  compared  with 
that  formed  by  a  prism. 

In  1802  Wollaston  detected  four  shaded  lines 
intersecting  the  spectrum  of  sunlight  at  right  angles 
to  its  length.  Bearing  in  mind  that  the  spectrum 
is,  in  reality,  a  series  of  pictures  of  the  slit,  one 
being  formed  by  each  kind  of  light  present  in  the 
mixture  submitted  to  analysis,  it  is  clear  that  the 
shaded  bands  must  indicate  colours  absent  from, 
or  feebly  represented  in,  the  light;  as  missing 
colours  in  sunlight.  Three  of  the  four  lines 
Wollaston  regarded  as  forming  the  natural  divisions 
between  different  shades  of  colour — at  best  a  not 
very  satisfactory  hypothesis,  and  one  shortly  to  be 
disproved. 

By  the  refinements  introduced  by  Fraunhofer  in 
1814,  the  number  of  dark  lines  in  the  solar  spectrum 
was  increased  to  close  upon  six  hundred,  and  it 

rM620)  L 


162          Recent  Advances  in  Astronomy. 

may  be  added  that  in  a  fine  modern  spectroscope 
upwards  of  ten  thousand  are  visible.  By  rotating 
the  observing  telescope  until  the  more  conspicuous 
of  them  were  brought  in  succession  to  the  centre 
of  the  field  of  view,  Fraunhofer  was  able,  from  the 
reading  of  a  graduated  circle  attached  to  the  tele- 
scope, to  measure  the  positions  of  the  lines  in  the 
spectrum,  or,  more  definitely,  the  degree  of  re- 
frangibility  of  each  missing  colour.  From  these 
measurements  he  constructed  the  first  map  of  the 
solar  spectrum,  denoting  the  more  conspicuous 
lines  by  the  capital  letters  from  A  to  H.  The  dark 
lines  are  still  known  as  the  Fraunhofer  lines,  and 
are  identified  by  Fraunhofer's  nomenclature. 

The  lines  indicated  by  A  and  B  upon  Fraunhofer's 
map  are  situated  in  the  deep-red  of  the  spectrum, 
and  are,  under  ordinary  conditions,  both  strongly 
marked,  c  is  a  sharp  well-defined  line  in  the 
region  of  the  spectrum  where  the  red  is  passing 
into  orange;  D,  a  close  pair  of  lines  in  the  yellow; 
E,  a  condensed  group  in  the  bright-green;  F,  a 
sharply-defined  line  in  the  greenish -blue;  G,  a 
group  in  the  deep-blue ;  and  H,  a  rather  wide  pair 
of  broad  diffused  lines,  or  rather  bands,  that  lie 
in  the  extreme  violet.  This  pair  have  assumed 
great  importance  in  recent  researches,  and  are  now 
generally  known  as  H  and  K. 

For  the  Fraunhofer  lines  to  appear  in  the  spec- 
trum, the  slit  must  be  very  narrow.  If  it  is  opened 
beyond  a  certain  degree  of  fineness,  the  images 
of  the  slit  formed  by  the  colours  lying  in  the  spec- 
trum on  either  side  of  the  position  of  the  colour, 


The  Analysis  of  Sunlight.  103 

to  the  absence  of  which  the  dark  line  is  due,  expand 
across  its  place  and  mask  the  effect  of  its  absence. 

Observations  of  the  very  impure  spectra  obtained 
by  viewing  flames  directly  through  a  prism  had 
already  been  made  by  different  observers  with 
but  little  result,  but  now  Fraunhofer  subjected 
their  light  to  examination  in  his  more  refined 
spectroscope.  With  no  flames  were  any  dark  lines 
observed  in  the  spectra,  but  bright  lines  frequently 
made  their  appearance,  shining  out  conspicuously 
upon  the  background  of  a  continuous  spectrum. 
The  bright  lines  clearly  pointed  to  the  presence, 
in  the  light  under  examination,  of  strongly  devel- 
oped pure  colours  that  were  not  decomposed  by 
the  prism.  In  all  flames  that  were  examined,  and 
especially  strongly  represented  in  the  blue  base  of 
a  candle-flame,  were  two  nearly  coincident  shades 
of  yellow,  revealed  by  the  appearance  in  the  spec- 
trum of  two  closely  adjacent  yellow  lines.  Fraun- 
hofer remarked  with  astonishment  that  these  yellow 
lines  coincided  exactly  in  their  positions  in  the 
spectrum  with  the  two  components  of  the  double 
dark  line  that  he  had  already  distinguished  by  the 
letter  D  in  the  spectrum  of  the  Sun.  The  two 
shades  of  yellow,  so  abnormally  abundant  in  the 
light  of  a  candle,  appeared,  therefore,  to  be  absent 
from,  or,  at  any  rate,  but  feebly  represented  in,  the 
light  of  the  Sun. 

Later,  in  1823,  Fraunhofer  examined  the  spectra 
of  the  brighter  stars.  In  all  of  them  he  recognized 
dark  lines,  but  although,  in  a  few  instances,  the 
spectra  appeared  to  be  similar  to  that  of  the  Sun 


164          Recent  Advances  in  Astronomy. 

in  the  distribution  and  in  the  relative  intensity 
of  the  lines,  in  the  greater  number  they  were 
essentially  different.  The  spectrum  of  Sirius,  in 
particular,  displayed  only  three  dark  lines,  but  all 
were  far  broader  and  more  strongly  marked  than 
any  recognized  in  the  spectrum  of  the  Sun. 

For  these  as  well  as  other  reasons,  Fraunhofer 
strongly  maintained  that  the  dark  lines  denoted 
colours  initially  absent  in  the  radiation  of  the  Sun 
and  stars  themselves,  and  that  they  did  not  originate, 
as  had  been  suggested,  either  in  the  atmosphere 
of  the  Earth,  or  by  some  optical  effect  in  the 
spectroscope,  similar  to  that  by  which  dark  diffrac- 
tion lines  appear  when  an  illuminated  slit  is  viewed 
from  some  distance  through  a  second  slit,  a  pheno- 
menon that  was  at  that  time  attracting  considerable 
attention. 

The  origin  of  the  bright  lines  in  the  spectra  of 
flames :  the  source  of  the  dark  lines  in  the  spectra  of 
the  Sun  and  stars :  and  the  remarkable  coincidence 
established  by  Fraunhofer  between  the  two  yellow 
rays  emitted  by  flames  and  the  D  pair  absent  in  the 
light  of  the  Sun,  aroused  the  highest  interest  and 
evoked  the  keenest  inquiry.  For  many  years  the 
exact  study  of  the  bright  lines  in  flame  spectra 
made  but  little  progress.  It  was  found  that,  upon 
saturating  the  wick  of  a  flame  with  different 
chemicals,  with  the  exception  of  the  yellow  pair 
that  was  always  present,  different  sets  of  bright 
lines  appeared  in  the  spectrum.  It  appeared  pro- 
bable that  the  colours  represented  by  the  bright 
lines  were  emitted  by  the  glowing  vapours  of  the 


The  Analysis  of  Sunlight.  165 

substances  introduced  into  a  flame,  and  that  the 
continuous  spectrum  upon  which  they  appeared  was 
due  to  the  normal  radiations  of  the  flame  itself,  for, 
with  the  scarcely  luminous  flame  of  burning  alcohol, 
the  continuous  spectrum  nearly  disappeared,  and 
the  bright  lines  that  flashed  out  upon  the  introduc- 
tion of  various  chemicals  seemed  as  if  separated 
by  intervals  of  almost  complete  darkness.  The 
demonstration  of  the  now  familiar  fact  that  each 
glowing  vapour  emits  definite  colours,  indicated  by 
bright  lines  occupying  definite  positions  in  the 
spectrum,  and  that  from  the  appearance  of  its 
spectral  lines  the  presence  of  an  element  may  be 
inferred  with  certainty,  was,  however,  only  estab- 
lished by  the  classical  researches  of  Bunsen  and 
Kirchhoff  in  1859. 

But,  in  the  meantime,  facts  of  great  interest  were 
brought  to  light  in  connection  with  the  dark 
Fraunhofer  lines.  In  1832  Sir  David  Brewster 
noticed  that  as  the  Sun  approached  the  horizon 
many  of  the  Fraunhofer  lines  became  intensified. 
Several  groups  of  lines  towards  the  red  end  of  the 
spectrum,  for  instance,  while  delicately  defined 
when  the  Sun  is  high,  appear  at  sunrise  and  sunset 
as  massive  black  columns  standing  in  front  of  the 
deep-red  of  the  spectrum.  Since,  when  at  a  low 
altitude,  the  rays  of  the  Sun  penetrate  the  atmos- 
phere obliquely,  and  their  path  included  in  the  air 
is  therefore  very  great,  Brewster  suggested  that 
those  lines  that  were  affected  in  this  manner  were 
caused  by  absorption  by  the  Earth's  atmosphere  of 
the  colours  corresponding  to  them ;  their  intensifica- 


166          Recent  Advances  in  Astronomy. 

tion  with  the  low  sun  being  due  to  increased  absorp- 
tion by  reason  of  the  greater  atmospheric  path  of 
the  light.  The  truth  of  this  view  has  since  been 
abundantly  confirmed,  and  the  lines  that  thus 
originate  are  known  as  telluric  lines.  The  great 
majority  of  the  Fraunhofer  lines  appeared,  however, 
to  be  independent  of  the  atmospheric  track  of  the 
solar  rays,  since  they  were  not  affected  in  intensity 
by  the  altitude  of  the  Sun,  and  it  was  therefore 
assumed  that  they  denoted  colours  absent  in  sun- 
light before  it  entered  the  Earth's  atmosphere. 
They  were,  in  consequence,  regarded  as  owing 
their  origin  to  a  similar  absorption  of  definite 
colours  in  an  atmosphere  that  was  supposed  to 
envelop  the  incandescent  surface  of  the  Sun. 

In  the  discovery  of  the  origin  of  the  telluric  lines, 
the  first  glimpse  was  obtained  of  the  remarkable 
power  possessed  by  many  gases  of  absorbing 
colours  so  definitely  as  to  more  or  less  completely 
extinguish  them  without  affecting  those  immediately 
on  either  side  of  them  in  the  spectrum.  In  the 
following  year  Brewster  showed  that,  instead  of 
invariably  necessitating  an  extensive  atmosphere  to 
produce  the  effect,  with  some  gases  a  few  inches 
were  sufficient;  for,  on  causing  the  light  from  a 
candle -flame  to  traverse  such  small  lengths  of 
certain  gases  before  entering  the  slit  of  the  spectro- 
scope, the  spectrum  became  ruled  throughout  by 
dark  lines  and  shaded  bands.  Upon  introducing  a 
glass  tube  filled  with  the  ruddy  vapour  of  nitric 
peroxide  between  a  lamp -flame  and  a  spectroscope, 
the  spectrum  instantly  became  crossed  by  an 


The  Analysis  of  Sunlight.  167 

enormous  number  of  dark  lines,  some  broad  and 
massive,  and  others  most  delicately  fine.  Each 
dark  line  denoted  the  absence  of  a  definite  colour 
that  had  been  absorbed  by  the  vapour.  Some  of 
the  lines  appeared  to  coincide  in  their  positions  in 
the  spectrum  with  certain  of  the  Fraunhofer  lines 
in  the  spectrum  of  the  Sun,  from  which  Brewster 
was  led  to  conclude  that  nitric  peroxide  was  a  con- 
stituent of  the  Sun's  atmosphere.  Although  this 
conclusion  has  been  disproved,  in  suggesting  it 
Brewster  obtained  the  first  glimpse  of  one  of  the 
most  powerful  and  remarkable  of  the  methods  ot 
modern  scientific  analysis. 

The  years  that  immediately  followed  Brewster's 
observations  marked  the  birth  of  the  science  of 
photography.  In  1838  Daguerre  had  discovered 
the  process,  with  which  his  name  has  been  since 
associated,  for  causing  objects,  by  means  of  their 
light  radiations,  to  impress  their  pictures  upon 
specially  prepared  silver  surfaces;  in  1840  Dr. 
Draper  had  effected  the  first  application  of  the  dis- 
covery to  astronomy  in  photographing  the  Moon ; 
and  two  years  later  Becquerel  succeeded  in  obtain- 
ing a  photograph  of  the  solar  spectrum  by  project- 
ing it  upon  a  sensitive  plate.  In  this  first  photo- 
graph of  the  spectrum  the  dark  lines  appeared  as 
surely  as  in  eye  observation,  while  the  remarkable 
fact  became  apparent,  that  the  spectrum  did  not 
terminate  with  the  violet,  but  extended  beyond  it 
to  a  distance  far  exceeding  its  visible  limits,  con- 
tinuing, in  its  invisible  extension,  to  be  crossed  by 
lines  of  absent  radiation.  It  appeared,  therefore, 


168          Recent  Advances  in  Astronomy. 

that  the  total  radiation  of  the  Sun  contains  rays 
more  refrangible  than  violet  light,  and  which  do 
not  possess  the  power  of  exciting  the  sense  of 
vision.  A  year  later,  Draper,  also  by  the  aid  of 
photography,  similarly  traced  the  solar  spectrum 
beyond  its  visible  limit  in  the  red,  and  there  also 
found  Fraunhofer  lines  of  absent  radiation. 

The  time  was  now  approaching  when  a  successful 
attack  was  to  be  made  upon  the  great  mystery  of 
the  Fraunhofer  lines.  In  1849  M.  Leon  Foucault 
devised  an  experiment  with  the  view  of  determining 
whether  or  not  the  coincidence — as  regards  position 
in  the  spectrum — between  the  two  components  of 
the  Fraunhofer  D  line  and  the  remarkable  close 
pair  of  yellow  lines  that  appeared  in  the  spectrum 
of  almost  every  flame,  was  exact.  In  this  experi- 
ment, which  has  become  classical,  the  yellow  pair 
were  obtained  from  the  light  of  the  electric  arc. 
The  electric  arc  is  formed  by  passing  a  current  of 
electricity  across  the  space  separating  the  ends  of 
two  carbon  rods  that  are  almost  in  contact.  In 
passing  through  the  rods  the  current  experiences 
but  little  resistance,  and  therefore  develops  but 
little  heat;  but  in  its  passage  from  one  rod  to  the 
other  across  the  air-gap  separating  them  enormous 
heat  is  developed  owing  to  the  greatly  increased 
resistance,  and  the  air  is  raised  to  a  very  high 
temperature.  The  intensely  hot  bridge  of  air  be- 
tween the  ends  of  the  rods  is  technically  known  as 
the  "arc  ".  The  temperature  of  the  arc  is  so  high 
that  impurities  present  in  the  carbon  rods,  and 
indeed  the  carbon  itself,  volatilize  and  mix,  in  the 


The  Analysis  of  Sunlight.  169 

state  of  gas,  with  the  air  in  the  gap.  In  spite  of  its 
high  temperature,  the  arc  itself  gives  but  little 
light,  owing  to  the  poor  radiating  power  of  the 
gases  forming  it;  but  the  ends  of  the  rods,  bathed 
in  these  highly-heated  gases,  are  raised  to  the 
vivid  state  of  incandescence  that  is  the  source 
of  light  in  the  arc  lamp.  The  electric  arc  had 
been  discovered  by  Sir  Humphry  Davy  in  the  year 
1800. 

On  directing  the  spectroscope  toward  either  of 
the  incandescent  carbon  ends,  Foucault  observed 
that  the  light  agreed  with  that  radiated  by  all  in- 
candescent solid  bodies  in  yielding  a  continuous 
spectrum,  but  on  deflecting  the  spectroscope  towards 
the  gap,  so  that  the  light  from  the  glowing  gases 
should  be  subjected  to  analysis,  a  number  of  separ- 
ated bright  lines  were  seen,  indicating  the  existence, 
in  the  radiations  of  the  glowing  gases  of  the  arc,  of 
a  corresponding  number  of  isolated  colours.  Among 
the  lines  so  seen  the  familiar  yellow  pair  shone  out 
conspicuously.  To  test  whether  these  coincided 
exactly  in  their  spectral  position  with  the  components 
of  the  D  line  in  the  solar  spectrum,  Foucault  con- 
densed the  rays  from  the  Sun  upon  the  arc  by  means 
of  an  ordinary  condensing  lens.  The  solar  rays, 
after  traversing  the  arc,  streamed  onward,  and 
entered  the  slit  of  the  spectroscope  along  with  the 
rays  of  the  arc  itself,  and  Foucault,  anticipating 
that  the  coincidence  between  the  positions  of  the 
yellow  lines  and  the  D  lines  would  prove  to  be 
exact,  confidently  expected  that,  in  the  light  of  the 
Sun,  poor  in  definite  yellow  rays,  supplemented  by 


170          Recent  Advances  in  Astronomy. 

that  of  the  arc,  abnormally  rich  in  them,  the  D  lines 
would  altogether  disappear. 

On  observing  the  spectrum,  however,  Foucault 
witnessed  a  most  remarkable  and  unexpected  appear- 
ance. Not  only  were  the  dark  D  lines  not  filled  in 
by  the  yellow  lines  of  arc  spectrum,  but  theyappeared 
to  be  both  darker  and  wider  than  when  the  arc  was 
absent.  Not  only  did  the  radiations  of  the  arc  fail 
to  supplement  the  deficiency  of  the  similar  radiations 
in  sunlight,  but  the  deficiency  at  once  became  more 
pronounced  than  before.  Only  one  explanation 
appeared  possible.  Not  only  were  the  gases  of  the 
arc  capable  of  radiating  two  definite  shades  of  yel- 
low, but  they  also  possessed  the  power  of  absorbing 
them.  The  gases  of  the  arc  had  absorbed  a  greater 
quantity  of  the  yellow  rays  from  the  solar  radiation 
than  they  had  added  to  it,  and  increased  darkness 
had  been  the  result.  Further,  excluding  the  Sun 
altogether,  Foucault,  by  the  aid  of  a  mirror,  re- 
flected the  light  from  one  of  the  incandescent  carbon 
points  through  the  gases  occupying  the  gap  be- 
tween the  pair;  and  on  submitting  the  light  thus 
transmitted  to  analysis,  observed  a  continuous  spec- 
trum crossed  by  a  pair  of  fine  dark  lines  in  the 
yellow.  The  arc  had  again  absorbed  the  two 
shades  of  yellow  more  abundantly  than  it  had 
radiated  them,  and  the  D  lines  had  been  produced 
for  the  first  time  in  a  laboratory  experiment. 

There  can  be  little  doubt  that  if  the  origin  of  the 
yellow  pair  had  been  known,  the  problem  of  the 
Fraunhofer  lines  would  have  found  its  solution  in 
Foucault's  experiments.  The  yellow  lines  had, 


The  Analysis  of  Sunlight.  171 

however,  proved  a  veritable  stumbling-block  to  the 
advance  of  spectrum  analysis.  In  the  greater  num- 
ber of  cases  it  seemed  probable  that  the  appearance 
of  definite  bright  lines  in  spectra  depended  upon 
the  presence  of  definite  glowing  vapours  in  the 
source  of  light,  but  the  yellow  pair  seemed  to  defy 
any  such  limitation.  They  flashed  out  in  the 
spectra  of  all  flames,  they  seemed  to  be  associated 
with  the  burning  of  all  substances;  and  it  was 
indeed  suggested  that  they  were  developed  in,  and 
inseparably  connected  with,  the  process  of  com- 
bustion. For  a  few  years  after  Foucault's  obser- 
vations they  succeeded  in  evading  the  most  refined 
methods  of  scientific  inquiry.  By  the  year  1852, 
however,  Sir  Gabriel  Stokes  had  shown  that  they 
were  absent  from  the  spectrum  of  a  candle-flame 
when  the  wick  had  been  carefully  snuffed  clean  and 
so  as  not  to  project  into  the  luminous  envelope,  as 
well  as  from  the  spectrum  of  the  flame  of  pure 
alcohol  when  burned  in  a  carefully-cleaned  watch- 
glass.  On  the  other  hand,  they  were  most  intensely 
developed  when  common  salt — the  chloride  of  so- 
dium— and  other  compounds  of  sodium  were  intro- 
duced into  flames.  Gradually  it  became  more  and 
more  probable  that  they  were  due  to  the  glowing 
vapour  of  sodium,  and  that  their  almost  universal 
appearance  in  spectra  arose  from  the  extreme  diffi- 
culty of  excluding  a  last  trace  of  salt,  and  from  their 
very  powerful  development  upon  the  presence  of 
the  smallest  possible  quantity  of  it.  Assuming 
this  to  be  the  explanation  of  their  appearance,  Sir 
Gabriel  Stokes,  in  1852,  gave  the  correct  explan- 


172          Recent  Advances  in  Astronomy. 

ation  of  the  appearance  of  the  D  lines  in  the  spectrum 
of  the  Sun. 

Sir  Gabriel  Stokes's  explanation  was  based  upon 
theoretical  grounds — the  wave  theory  of  light,  and 
the  view  of  the  structure  of  matter  involved  in  its 
acceptance.  Since,  in  its  later  history,  the  most 
important  applications  of  the  analysis  of  light  to 
astronomy  have  been  directly  due  to  the  view  of 
the  nature  of  light  indicated  in  the  wave  theory,  it 
may  be  well  to  make  a  slight  digression  in  a  short 
sketch  of  its  general  features. 

According  to  the  wave  theory  of  light — originally 
enunciated  by  Christian  Huygens  in  the  latter  part 
of  the  seventeenth  century,  suppressed  for  a  time 
by  the  overpowering  authority  of  Sir  Isaac  Newton, 
but  placed  upon  a  sound  scientific  foundation  early 
in  the  present  century  by  the  labours  of  Dr.  Thomas 
Young — light  is  due  to  the  transmission  of  waves, 
or  undulations,  from  a  luminous  body  to  the  eye. 
For  there  to  be  undulations  there  must  be  some- 
thing to  undulate,  and  to  this  something  the  name 
has  been  given  of  the  "  ether".  To  account  for 
the  phenomena  of  light,  it  is  necessary  to  regard 
the  ether,  as  not  only  existing  throughout  space,  at 
any  rate  to  the  farthest  of  the  visible  stars,  but  as 
permeating  all  matter. 

The  display  of  iridescent  colour  frequently  so 
exquisitely  developed  in  light  reflected  from  thin 
films,  such  as  the  envelope  of  a  soap-bubble;  the 
coloured  fringes  visible  upon  either  side  of  an 
illuminated  slit  when  viewed  through  a  second  and 
similar  slit  at  a  moderate  distance  from  it;  and  the 


The  Analysis  of  Sunlight.  173 

spectrum  formed  by  a  diffraction  grating,  enable 
us,  if  we  interpret  them  according  to  the  wave 
theory — by  which  alone  they  have  so  far  received 
a  satisfactory  explanation — to  measure  the  lengths 
of  the  ether  waves.  Estimates  deduced  from  the 
different  phenomena  are  in  perfect  accord,  though 
there  is  no  doubt  that  the  highest  degree  of  accuracy 
is  obtainable  in  observations  of  the  diffraction  spec- 
trum. From  them  it  appears  that  colour,  and  there- 
fore refrangibility,  is  determined  by  the  length  of 
waves  in  free  ether;  the  sensation  of  red  being 
excited  by  the  longest  and  that  of  violet  by  the 
shortest  waves  that  affect  the  eye,  while  the  pass- 
age up  the  spectrum  from  red  to  violet  is  accom- 
panied by  a  continual  decrease  of  wave-length. 
The  actual  wave-lengths  are  about  a  sixty-thou- 
sandth of  an  inch  for  violet,  and  a  thirty-thousandth 
of  an  inch  for  red  light,  but  they  are  more  accu- 
rately given  in  the  table  on  p.  175. 

It  will  scarcely  be  necessary  to  remind  the  reader 
that  the  appearance  of  the  transmission  of  matter  by 
wave  motion  is  illusory.  On  the  surface  of  water 
disturbed  by  wave  motion  floating  objects  merely 
rise  and  fall  as  the  waves  pass,  which  shows  that 
the  movement  of  the  wave-conveying  medium  con- 
sists of  a  succession  of  oscillations  up  and  down, 
while  the  waves  themselves  continually  pass  them 
horizontally.  The  illusion  of  water  moving  with 
waves  results  from  each  portion  of  the  surface  trans- 
mitting its  disturbance  to  the  portion  immediately 
in  front  of  it,  but  occupying  a  definite  interval  of 
time  in  so  doing.  After  a  short  interval,  therefore, 


174          Recent  Advances  in  Astronomy. 

the  form  of  the  water  surface  has  been  moved  for- 
ward, but  so  continuously  that  the  appearance  is 
produced  of  the  surface  itself  having  been  displaced 
in  the  direction  of  the  wave  motion. 

The  double  appearance  of  objects  when  viewed 
through  Iceland  spar  and  other  crystals,  as  well  as 
the  chromatic  effects  and  general  properties  of 
polarized  light,  indicate  that  the  motion  of  the  ulti- 
mate parts  of  the  wave-conveying  ether  is  transverse, 
or  across  the  direction  of  wave  motion.  In  this 
respect  ether  waves  resemble  waves  upon  the  sur- 
face of  water,  as  well  as  those  upon  stretched 
strings.  They  differ  in  character  from  waves  of 
sound,  in  that  in  these  the  motion  of  the  air — the 
undulating  medium  in  their  case — consists  of  oscil- 
lations to  and  fro  in  the  direction  of  wave  motion. 

During  the  passage  of  a  single  wave  past  a  point 
in  the  ether,  the  ether  at  the  point  executes  a  single 
vibration  or  oscillation  about  its  normal  position, 
this  vibration  being,  according  to  the  inference  of 
the  preceding  paragraph,  across  the  direction  of 
wave  motion.  During  the  passage  of  a  train,  or 
series,  of  similar  waves,  the  ether,  therefore,  con- 
tinues to  oscillate,  and  the  number  of  oscillations 
executed  every  second — a  quantity  known  as  the 
"  frequency  of  oscillation" — is  determined  by,  and 
is  equal  to,  the  number  of  waves  passing  in  a 
second.  Since  all  waves  travel  with  the  same  speed, 
the  longer  will  pass  in  less  rapid  succession  than 
the  shorter,  and  will  therefore  produce  a  less  rapid 
oscillation.  The  violet  waves,  for  instance,  being 
only  about  half  as  long  as  the  red,  are  associated 


The  Analysis  of  Sunlight. 


'75 


with  double  the  frequency  of  oscillation.  From  the 
geometry  of  wave  motion  thus  sketched  it  follows 
from  simple  reasoning  that  in  every  case  the  velocity 
of  the  waves  is  equal  to  the  product  of  their  length 
into  the  frequency  of  oscillation.  From  this  rela- 
tion it  is  possible  to  determine  the  frequency  of 
oscillation  of  different  kinds  of  light  waves,  since 
their  velocity — the  speed  of  light — is  known,  and 
their  length  may  be  determined  in  every  case  by  a 
diffraction  grating.  This  has  been  carried  out  in 
the  following  table,  the  speed  of  light  being  taken 
as  187,000  miles  per  second.  The  way  in  which 
the  frequency  of  oscillation  increases  as  the  wave- 
length decreases  should  be  carefully  noticed. 

WAVE-LENGTHS  AND  FREQUENCIES  OF  OSCILLATION  OF 
ETHER  WAVES. 


Colour. 

Fraunhofer 
Line. 

Wave-length 
in  free  ether,1 
in  millionths 
of  an  inch. 

Vibrations  per 
second  in 
millions  of 
millions. 

Red  

A 

29-9 

395-8 

Orange.  .        ... 

6 

c 

27-0 

2C'Q 

437 
4<;8 

Yellow 

D 

2V2 

cio 

Green         .  . 

E 

2O  '7 

j1^ 
C.7Q 

Blue 

F 

I9'I 

618 

Violet  

G 
H 

17*3 
IS  '6 

683 

71:7 

xThe  necessity  for  the  addition  of  the  words  "in  free  ether"  is  due  to  the 
fact  that,  while  traversing  transparent  substances,  the  speed  of  light  is 
reduced,  doubtless  as  the  result  of  the  close  association  of  the  ether  with 
ordinary  matter.  The  frequency  of  oscillation  must,  however,  remain  the 
same,  being  always  that  of  the  source  of  the  waves,  so  that  the  relation — 
speed  of  waves  =  wave-length  x  frequency — indicates  that  the  wave-length 
must  be  reduced  in  proportion  to  the  speed.  It  is  customary  to  define  a 
particular  kind  of  light  by  its  wave-length.  Since,  however,  this  varies  with 
the  medium  through  which  the  light  travels,  it  would  be  far  better  to  define 


176          Recent  Advances  in  Astronomy. 

The  most  simple  view  to  take  with  reference  to 
the  generation  of  waves  in  the  ether,  is  that  the 
luminous — or  wave-generating" — body  contains  os- 
cillating portions  of  matter  that  possess  a  grip 
upon  the  ether.  As  the  oscillations  of  a  hand  or 
tuning-fork  may  develop  waves  upon  a  stretched 
cord,  so  these  vibrating  parts,  gripping  the  ether, 
and  being  thus  able  to  transmit  their  movement  to 
those  portions  of  it  immediately  round  them,  set  up 
waves,  the  frequency  of  oscillation  of  which  is  the 
same  as  their  own.  What  these  oscillating  parts 
are  is  not  of  fundamental  importance  to  the  present 
study,  but  they  are  generally  regarded  either  as  the 
atoms  of  matter,  or  as  portions  of,  or  structures  in, 
the  atoms,  elastically  attached  and  capable  of  oscil- 
lation within  them.  The  atom  of  sodium,  capable 
of  emitting  two  yellow  rays  of  nearly  the  same  tint, 
that  is,  of  developing  two  series  of  waves  of  nearly 
the  same  frequency,  may  be  regarded  as  analogous 
to  a  musical  instrument  with  only  two  strings  tuned 
nearly  to  unison.  Since  the  waves  generated  by 
the  incandescent  vapour  of  sodium  cause  the  ether 
to  oscillate  about  510  millions  of  millions  of  times 
in  a  second,  this  rate  of  vibration  must  also  be  that 
of  the  structures  themselves.  Below  the  tempera- 
ture at  which  the  yellow  light  is  emitted  we  must 
suppose  that  the  structures  are  not  oscillating  with 

it  by  its  frequency  of  oscillation.  When  light  is  defined  by  its  wave-length, 
that  in  free  ether  should  always  be  understood.  In  glass  the  speed  of  light 
is  about  %  of  its  speed  in  free  ether.  In  air  it  is  scarcely  affected.  Re- 
fraction is  the  direct  result  of  alteration  of  speed  in  passing  from  one 
medium  into  another,  and  from  the  extent  of  refraction  the  alteration  in 
speed,  and  therefore  in  wave-length,  can  be  determined. 


The  Analysis  of  Sunlight.  177 

this  frequency,  but  that  the  oscillations  may  be 
developed  by  sufficient  addition  of  heat.  The 
higher  the  temperature,  the  more  intense  the  radia- 
tions, and  therefore  the  more  intense  the  oscilla- 
tions of  the  structures.  The  reader  is  probably 
aware  that  there  is  good  reason  for  regarding  such 
atomic  and  molecular  vibration  as  constituting  the 
sensible  heat  of  matter. 

Adopting  this  view  as  representing  the  mechan- 
ism of  radiation,  that  of  absorption  follows  naturally. 
Upon  a  series  of  ether  waves  traversing  a  space 
throughout  which  atoms  of  matter  are  distributed, 
the  atomic  structures  gripping,  and  therefore 
gripped  by,  the  ether,  will  tend  to  be  thrown  to 
and  fro  in  harmony  with  its  movement.  Heat  will 
be  represented  by  the  motion  generated  in  the 
atoms,  while  the  energy  of  the  waves  themselves 
will  be  correspondingly  decreased  by  the  loss  of  the 
motion  transferred  from  them  to  the  matter. 

There  is,  however,  one  case  in  which  the  trans- 
ference of  the  energy  of  motion  from  the  ether 
waves  to  the  atoms  will  be  especially  pronounced. 
It  is  a  familiar  fact,  deducible  from  the  first  prin- 
ciples of  mechanics,  that,  if  a  body  capable  of 
independent  vibration  is  acted  upon  by  a  succes- 
sion of  impulses  acting  in  unison  with  its  own 
oscillations,  a  far  more  extensive  oscillation  will 
result  than  if  such  a  coincidence  did  not  exist.  The 
principle,  generally  known  as  that  of  sympathetic 
vibration  or  resonance,  is  abundantly  illustrated 
throughout  mechanics  and  physics.  If  a  weight 
be  suspended  by  a  cord,  the  upper  end  of  which  is 

(M520)  M 


178          Recent  Advances  in  Astronomy. 

held  in  the  hand,  a  succession  of-  properly  timed, 
but  scarcely  appreciable,  movements  of  the  hand  to 
and  fro  may  cause  a  very  extensive  oscillation  of 
the  weight.  If  a  large  man  be  seated  in  a  garden 
swing,  a  little  man,  by  properly  timing  his  thrusts 
into  unison  with  the  oscillations  of  the  swing,  may 
develop  in  the  large  man  an  oscillation  out  of  all 
proportion  to  that  which  would  otherwise  result 
from  his  most  violent  efforts.  A  musical  note 
sounded  in  the  neighbourhood  of  a  jar  of  such 
dimensions  that  the  natural  period  of  oscillation  of 
the  contained  air  coincides  with  that  of  the  note, 
will  cause  the  air  of  the  jar  to  sound  loudly  in 
response.  The  recently  discovered  possibility  of 
electric  signalling  over  considerable  distances  with- 
out connecting  wires  depends  upon  the  coincidence 
between  a  succession  of  feeble  electric  impulses 
applied  to  a  distant  conductor,  and  the  normal 
oscillations  of  electricity  in  the  conductor, 

If,  now,  white  light — that  is,  a  number  of  wave- 
trains  of  all  possible  frequencies  between  the  limits 
of  the  spectrum — should  traverse  the  vapour  of 
sodium,  it  should  not  be  difficult  to  predict  what 
would  occur.  Those  waves,  the  frequencies  of 
which  did  not  agree  with  the  natural  vibrations  of 
the  sodium  atoms,  would  scarcely  affect  them, 
and  therefore  they  themselves  would  be  scarcely 
affected.  Those  waves,  however,  that  possessed 
vibration  frequencies  in  unison  with  the  normal 
oscillations  of  the  atoms,  would  apply  impulses  to 
the  atoms  or  atomic  structures  accurately  timed  to 
their  own  oscillations;  resonance  would  follow,  and 


The  Analysis  of  Sunlight.  179 

extensive  motion  would  be  developed  in  the  atoms. 
During  the  development  of  this  motion,  equal 
energy  of  motion  would  be  absorbed  from  the 
waves.  The  waves,  from  which  this  energy  would 
be  absorbed,  would  be  damped,  and  the  light,  after 
having  traversed  the  vapour,  would  be  found  defi- 
cient in  precisely  those  waves  that  the  vapour  itself 
could  originate.  Generally  any  vapour  possessing 
the  power  of  emitting  definite  radiations  must  also 
possess  a  special  capacity  for  absorbing  them. 

The  deficiency  in  sunlight  of  the  two  shades  of 
yellow  emitted  by  the  glowing  vapour  of  sodium, 
indicates,  therefore,  that  the  white  light  of  the  Sun 
has  traversed  the  vapour  of  sodium  somewhere  in 
its  passage  to  the  surface  of  the  Earth.  Since  the 
vapour  of  sodium  is  not  found  in  the  atmosphere  of 
the  Earth,  and  is  assuredly  not  distributed  in  inter- 
planetary space,  it  must  be  looked  for  in  the  atmos- 
phere of  the  Sun. 

Such  was  Sir  Gabriel  Stokes's  explanation  of  the 
double  line  D,  and  of  other  Fraunhofer  lines,  though 
up  to  that  time  no  other  coincidence  between  Fraun- 
hofer lines  and  bright  lines  in  the  spectra  of  glowing 
terrestrial  vapours  has  been  established.  With  the 
singular  modesty  and  reticence  that  has  character- 
ized him  through  life,  Stokes,  while  offering  one  of 
the  most  remarkable  of  scientific  theories  as  a  sug- 
gestion in  private  conversation  with  a  friend,  re- 
frained from  making  it  public.  In  the  following 
year  (1853),  however,  a  similar  explanation  was 
given  by  Angstrom  of  Upsala.  Angstrom,  more- 
over, established  the  coincidence  between  other  of  the 


i8o          Recent  Advances  in  Astronomy. 

Fraunhofer  lines  and  certain  bright  lines,  though  of 
unknown  origin,  in  the  spectrum  of  the  electric  arc. 
In  1859  tne  publication  of  the  classical  researches 
of  Bunsen  and  Kirchhoff  placed  spectrum  analysis 
upon  a  sound  foundation  as  a  branch  of  science. 
For  the  first  time,  bright  lines  in  the  spectra  of 
flames  were  definitely  proved  to  arise  from  the 
presence  of  glowing  vapours  in  the  flames.  The 
flame  generally  employed  was  that  of  a  spirit-lamp, 
or  of  gas  which  had  been  deprived  of  its  luminosity 
in  the  now  familiar  form  of  Bunsen  burner.  A 
great  number  of  different  substances  were  made  to 
pass  into  the  flame  in  the  state  of  vapour,  by  intro- 
ducing them  in  the  solid  or  liquid  state  upon  a 
piece  of  platinum  wire  into  the  lower  part  of  the 
flame.  It  was  found  that  every  system  of  bright 
lines  was  associated  with  the  presence  of  a  definite 
vapour  in  the  flame,  and  with  such  consistency 
that  the  presence  of  the  vapour  could  be  inferred 
with  certainty  from  the  appearance  of  its  character- 
istic lines  in  the  spectrum.  This  fact,  of  course, 
constitutes  the  very  foundation  of  spectrum  analysis. 
The  famous  close  yellow  pair  were  traced  to  sodium; 
and  it  was  shown  that  their  continual  appearance 
in  all  sorts  and  conditions  of  flames  was  due  to  the 
universal  distribution  of  common  salt  in  the  atmos- 
phere, carried  into  it  in  all  probability  in  the  first 
instance  by  sea  spray,  and  to  the  marvellous  de- 
licacy of  the  spectral  test,  a  delicacy  so  extreme 
that  the  yellow  lines  appeared  in  the  spectrum  on 
the  introduction  of  a  two-hundred-millionth  part  of 
a  grain  of  salt  into  the  flame. 


The  Analysis  of  Sunlight.  181 

Not  only  was  the  complete  set  of  bright  lines 
yielded  by  one  glowing  vapour  different  from  that 
given  by  any  other,  but  only  rarely  did  any  one  of 
the  lines  of  one  element  appear  to  occupy  in  the 
spectrum  the  position  of  a  line  of  another.  Whether 
the  few  coincidences  that  have  to  the  present  time 
been  observed  are  in  any  case  more  than  approxi- 
mate, the  result  of  insufficient  spectroscopic  power; 
whether  they  are  exact ;  and,  if  so,  whether  they  are 
more  than  accidental,  has  been  very  keenly  dis- 
cussed in  recent  years  in  connection  with  certain 
modern  speculations.  In  a  great  many  instances 
apparent  coincidences  have  been  shown,  by  the 
application  of  more  refined  instrumental  means,  to 
be  only  approximate,  and  the  view  is  now  generally 
held  that  the  few  as  yet  unresolved  will  either  yield 
to  higher  dispersion  or  are  merely  accidental. 

In  1859  the  splendid  results  obtained  by  Bunsen 
and  Kirchhoff  were  brought  to  bear  upon  the 
problem  of  the  Fraunhofer  lines  by  Kirchhoff. 
Kirchhoff  found  no  difficulty  in  obtaining  the 
sodium  lines  dark  upon  the  background  of  a  con- 
tinuous spectrum,  by  interposing  the  flame  of  a 
spirit-lamp,  upon  the  wick  of  which  a  few  grains 
of  salt  had  been  sprinkled,  in  the  path  of  rays  pro- 
ceeding from  the  incandescent  lime  of  the  lime- 
light to  the  slit  of  a  spectroscope.  Further,  he 
failed  to  obtain  the  effect  when  the  flame  of  a 
bunsen  burner  similarly  charged  with  salt  was  used 
instead  of  the  spirit-lamp,  but  perceived  instead  the 
bright-yellow  pair  radiated  by  the  flame  superposed 
upon  the  less  brilliant  continuous  spectrum  of  the 


182  Recent  Advances  in  Astronomy. 

lime-light.  From  this  he  suspected  that  to  effect 
"  reversal "  the  temperature  of  the  vapour  must  be 
less  than  that  of  the  radiating  source. 

The  statement  of  the  exact  conditions  under  which 
a  vapour  will  effect  the  reversal  of  its  spectral  lines 
was  first  given  by  Balfour  Stewart  in  1861.  In  1791 
Prevost  of  Geneva  had  published  a  suggestive  paper 
entitled  "  On  the  Equilibrium  of  Heat",  in  which, 
according  to  a  law  then  enunciated  for  the  first  time, 
and  since  known  as  "  the  law  of  exchanges  ",  it  was 
shown  that  every  body  when  at  the  same  tempera- 
ture as  those  surrounding  it,  must  possess  a  power 
of  absorbing  heat  radiations  in  direct  proportion  to 
its  power  of  emitting  them.  At  the  time  of  Kirch- 
hoff's  researches,  it  was  thought  to  be  probable, 
from  the  similarity  in  the  laws  by  which  they  were 
governed,  that  radiant  heat  and  light  were  different 
forms  of  one  kind  of  radiation,  and  Balfour  Stewart 
perceived  that  the  extension  of  Prevost's  reasoning 
to  light  radiation  would  account,  not  only  for  the 
fact  of  reversal,  but  also  for  the  condition,  already 
suggested  by  experiment,  that  to  effect  it,  the  ab- 
sorbing vapour  must  be  cooler  than  the  source. 

Limits  of  space,  unfortunately,  make  it  impossible 
to  introduce  Balfour  Stewart's  reasoning  here,  but 
an  excellent  outline  is  given  in  Balfour  Stewart's 
Heat,  and  will  well  repay  the  most  careful  study. 
By  a  very  simple  process  of  reasoning,  based  upon 
the  obviously  sound  assumption  that  a  body  within 
an  opaque  enclosure,  all  portions  of  the  inner  sur- 
face of  which  are  at  the  same  constant  temperature, 
will  ultimately  acquire  the  temperature  of  the  en- 


The  Analysis  of  Sunlight.  183 

closure,  Balfour  Stewart  showed  that  such  a  body, 
when  at  the  temperature  of  the  enclosure,  must  ab- 
sorb and  emit  any  particular  kind  of  radiation  in 
exactly  equal  amount.  Since  the  rate  of  radiation 
from  a  surface  is  directly  dependent  upon  tempera- 
ture, while  the  rate  of  absorption  depends  upon  the 
nature  of  the  surface  and  is  not  directly  affected  by 
its  temperature,  it  follows  that,  under  the  conditions 
imagined,  any  fall  in  the  temperature  of  the  body 
will  cause  its  radiation  to  fall  short  of  its  absorption, 
while  a  rise  in  temperature  will  cause  its  radiation 
to  exceed  the  absorption. 

From  this  conclusion  it  is  possible  to  state  defi- 
nitely an  essential  condition  for  the  appearance  of 
dark  lines  in  the  spectrum  of  the  Sun.  It  is,  that 
the  gases  present  in  the  solar  atmosphere  must  be 
at  a  lower  temperature  than  the  incandescent  sur- 
face— or  photosphere — behind.  If  the  temperature 
of  the  gases  were  equal  to  that  of  the  photosphere, 
they  would  be  in  such  a  condition  as  to  absorb  from 
its  light  precisely  as  much  of  any  kind  of  radiation 
as  they  would  add  to  it,  and,  in  consequence,  the 
light  from  the  photosphere  would,  after  traversing 
them,  be  unchanged  in  composition.  If  the  tem- 
perature of  the  atmosphere  were  to  fall  below  that 
of  the  photosphere,  the  radiation  of  its  gases  would, 
for  each  particular  kind  of  ray,  fall  short  of  the 
absorption,  and  dark  lines  would  result  in  the 
spectrum;  while,  if  the  temperature  of  the  atmo- 
sphere were  to  rise  above  that  of  the  photosphere, 
the  radiation  of  its  gases  would  exceed  the  absorp- 
tion exercised  by  them,  and  bright  lines  would 


184          Recent  Advances  in  Astronomy. 

appear  upon  the  continuous  spectrum  of  the  photo- 
sphere. From  the  general  appearance  of  dark  lines, 
the  Sun's  atmosphere  may  be  assumed  to  be  cooler 
than  the  photosphere,  though,  as  will  be  seen  later, 
bright  lines  do  occasionally  appear. 

It  is  clear  from  these  principles  that  the  Fraun- 
hofer  lines  are  not  absolutely  dark,  but  only  appear 
so  by  contrast  with  the  more  brilliant  spectrum  of 
the  photosphere  upon  which  they  are  projected. 
Even  if  a  gaseous  constituent  of  the  solar  atmo- 
sphere were  a  perfect  absorber  of  particular  kinds  of 
light,  its  own  radiation,  which  would  be  of  the  same 
nature  as  the  absorbed  light,  would  travel  on  with 
the  photospheric  rays  that  had  escaped,  and  would 
in  part  supply  the  place  of  those  that  had  been  ab- 
sorbed. There  is  no  doubt,  that  if  the  incandescent 
surface  of  the  Sun  were  for  a  moment  to  be  extin- 
guished, while  its  atmosphere  remained  unaffected, 
the  solar  spectrum  would  appear  as  a  crowd  of 
bright  lines  corresponding  to  actually  existing  dark 
ones.  The  existence  of  the  photosphere  behind 
does  not  detract  from  the  light  that  we  receive  from 
the  atmosphere,  but,  by  filling  the  spaces  between 
its  bright  lines  with  more  intense  light,  causes  them 
to  appear  dark  by  contrast.  That  the  apparently 
dark  Fraunhofer  lines  are  in  reality  brilliant  can  be 
shown  by  carefully-arranged  experiments. 

The  presence  of  sodium  in  the  atmosphere  of  the 
Sun  having  been  established,  Kirchhoff  next  en- 
deavoured to  discover  other  of  its  constituents,  by 
searching  in  the  spectra  of  terrestrial  elements  for 
bright  lines  that  should  coincide  in  their  positions 


The  Analysis  of  Sunlight.  185 

with  Fraunhofer  lines.  His  efforts  were  entirely 
successful.  On  passing  the  intense  electric  dis- 
charge of  an  induction  coil  from  one  metal  wire  to 
another  across  a  short  air  gap,  a  brilliant  spark  was 
obtained,  which,  when  analysed,  gave  a  spectrum 
of  bright  lines — clearly  due  to  the  glowing  gases 
filling  the  gap.  Different  sets  of  bright  lines  ap- 
peared as  different  metals  were  employed  to  form 
the  spark,  and  it  was  clear  that  the  spectrum  con- 
sisted of  the  bright  lines  given  by  incandescent  air, 
together  with  those  due  to  the  glowing  vapours  of 
the  metal  wires,  these  being  partially  volatilized  by 
the  intense  heat  of  the  discharge.  Passing  the  dis- 
charge between  the  ends  of  iron  wires,  the  spectrum 
given  was  that  of  a  mixture  of  the  vapour  of  iron 
and  air,  and  Kirchhoff  succeeded  in  establishing 
coincidences  between  no  fewer  than  sixty  of  the 
lines  that  were  due  to  the  iron  and  dark  lines  in  the 
spectrum  of  the  Sun.  It  was,  therefore,  proved  that 
the  vapour  of  iron  was  a  constituent  of  the  atmos- 
phere of  the  Sun. 

Continuing  his  researches  by  this  method,  Kirch- 
hoff considered  that  he  had  demonstrated  the  exist- 
ence in  the  atmosphere  of  the  Sun  of  nine  metals 
known  to  terrestrial  chemistry.  They  were — sodium, 
iron,  calcium,  magnesium,  nickel,  barium,  copper, 
zinc,  and  chromium.  Further,  it  was  regarded  as 
demonstrated,  from  the  absence  of  their  charac- 
teristic lines,  that  twelve  other  metals,  including 
gold,  silver,  and  mercury,  were  absent. 

In  1862  Angstrom  published  the  results  of  an 
extensive  series  of  observations,  similar  in  principle 


186          Recent  Advances  in  Astronomy. 

to  those  of  Kirchhoff,  on  the  chemistry  of  the  Sun's 
atmosphere.  Experimental  details  differed  from 
those  adopted  by  Kirchhoff  in  that  the  analysis  of 
the  light  was  effected  by  the  diffraction  grating, 
and  in  the  substitution  of  the  electric  arc  for  the 
discharge  of  an  induction-coil  as  the  source  of  heat. 
The  results  were  in  general  agreement  with  those 
of  Kirchhoff,  but  a  few  additional  elements  were 
detected,  among  which  the  most  interesting  was 
hydrogen.  The  spectrum  of  hydrogen  is  of  the 
highest  importance  in  the  astronomical  applications 
of  light  analysis.  When  enclosed  within  a  glass 
tube,  under  a  pressure  considerably  less  than  that 
of  the  atmosphere,  and  subjected  to  a  discharge  ot 
electricity — generally  led  into  and  from  the  gas  by 
platinum  wires  penetrating  the  glass — hydrogen 
gas  becomes  luminous,  emitting  a  soft  peach-like 
glow.  In  1859,  Plucker,  subjecting  this  glow  to 
analysis  in  the  spectroscope,  had  found  that  it  con- 
sisted in  the  main  of  three  bright  colours,  repre- 
sented in  the  spectrum  by  three  bright  lines — the 
first  of  a  magnificent  crimson,  the  second  of  a 
bluish-green,  and  the  third  of  a  deep-blue  colour. 
Angstrom  found  that  each  one  of  these  had  its 
counterpart  among  the  Fraunhofer  lines,  and  in 
1866  he  detected  a  fourth  line — of  a  violet  colour — 
in  the  hydrogen  spectrum,  and  found  that  it  also 
was -represented  by  a  dark  line  in  that  of  the  Sun. 
The  important  lines  c  and  F  in  Fraunhofer's  nomen- 
clature are  the  reversals  of  the  crimson  and  bluish- 
green  lines  of  hydrogen. 

It  is  unnecessary  to  give  more  than  the  briefest 


The  Analysis  of  Sunlight.  187 

outline  of  the  later  history  of  these  methods.  In 
1872  Sir  Norman  Lockyer  commenced  a  laborious 
series  of  comparisons  of  photographs  of  the  solar 
spectrum  with  those  of  spectra  of  metals  volatilized 
and  rendered  incandescent  by  the  electric  discharge 
of  an  induction-coil,  and,  as  the  result,  succeeded 
during  the  following  four  years  in  adding  about 
twenty  new  elements  to  the  fourteen  that  had  been 
previously  recognized  in  the  atmosphere  of  the  Sun. 
In  1887  Messrs.  Trowbridge  and  Hutchins  demon- 
strated the  existence  in  the  Sun  of  the  vapour  of 
carbon,  the  first  element  of  a  non-metallic  nature 
that  had  been  found  in  its  atmosphere;  while  in 
1891,  also  from  photographic  comparisons,  silicon 
was  detected  by  Professor  Rowland.  Some  idea 
of  the  fulness  of  detail  shown  in  Rowland's  photo- 
graphs may  be  gained  from  the  fact  that  coinci- 
dences were  established  in  them  of  upwards  of  two 
thousand  of  the  Fraunhofer  lines  and  bright  lines 
in  the  spectrum  of  iron. 

By  the  series  of  researches  that  have  been  traced, 
culminating  in  the  work  of  Kirchhoff,  spectrum 
analysis  was  raised  into  the  position  of  an  exact 
science.  It  appeared  to  be  all-powerful  in  problems 
to  which  the  application  of  its  methods  was  possible. 
Every  glowing  gas  was  regarded  as  emitting  and 
absorbing  definite  radiations.  By  the  appearance 
of  their  radiations  in  the  spectroscope  gases  could 
be  detected  with  certainty;  and  it  was  at  first 
not  unnaturally  concluded  that  by  the  absence  of 
their  radiations  from  glowing  matter  their  own 
absence  could  be  asserted  with  equal  confidence. 


i88          Recent  Advances  in  Astronomy. 

Had  this  anticipation  been  realized,  the  determina- 
tion of  the  presence  or  absence  of  any  terrestrial 
element  or  compound  in  the  atmospheres  of  the 
Sun  and  stars  would  have  been  only  a  matter  of 
careful  and  sufficiently  prolonged  observations,  and 
the  later  course  of  physical  astronomy  would  have 
been  strangely  different  from  its  actual  history. 

From  about  ten  years  after  the  date  of  Kirchhoffs 
work,  it  has  become  increasingly  apparent,  that, 
although  in  every  case  the  presence  of  a  glowing 
gas  is  directly  demonstrated  by  the  appearance  of  its 
characteristic  radiations,  some  caution  is  necessary 
before  the  absence  of  the  gas  can  be  inferred  with 
equal  certainty  from  the  absence  of  those  of  its  radia- 
tions with  which  we  are  familiar.  It  has  been  found 
that  change  of  physical  condition,  change  that  may 
result  from  alteration  of  temperature  or  pressure,  or 
even  from  the  admixture  of  other  substances,  may 
cause  the  familiar  radiations  of  a  gas  to  disappear 
and  to  be  replaced  by  others,  frequently  to  such  an 
extent  that  its  spectrum  may  assume  an  entirely 
new  and  unfamiliar  character,  in  which  no  relation 
to  its  former  self  is  apparent.  Of  the  many  instances 
of  such  modifications  of  spectra  that  have  been 
studied,  attention  may  be  specially  directed  to  two, 
both  of  great  importance  in  astronomical  physics. 

We  have  seen  that  the  visible  spectrum  of  hydro- 
gen consists  in  the  main  of  three  bright  lines — a 
crimson,  a  green,  and  a  blue — while  there  is  a  deeper 
violet  fourth  that  appears  under  strong  electrical 
excitement.  Of  these,  the  crimson  is  the  one  that 
appeals  most  strongly  to  the  eye.  In  1869  Sir 


The  Analysis  of  Sunlight.  189 

Edward  Frankland  and  Sir  Norman  Lockyer  made 
some  remarkable  observations  upon  these  lines  and 
those  of  nitrogen.  Hydrogen  gas  was  first  enclosed 
in  a  glass  tube  through  which  an  electric  current 
was  transmitted,  and  was  then  rarefied  by  the  action 
of  an  air-pump  connected  with  the  tube.  The  gas 
within  the  tube  became  incandescent  under  the  in- 
fluence of  the  current,  and  as  rarefaction  proceeded, 
the  lines  of  its  spectrum  became  finer  and  more 
brilliant,  and  after  a  time  a  limit  was  reached  at 
which  the  first  three  were  most  conspicuous.  The 
gas  was  then  subjected  to  a  further  rarefaction  while 
the  electric  discharge  was  maintained  at  a  moderate 
intensity.  During  the  progress  of  the  rarefaction 
the  spectrum  was  carefully  observed,  and  it  was 
seen  to  undergo  a  striking  alteration.  The  crimson 
and  blue  lines  became  fainter,  although  the  green 
line  was  scarcely  affected,  while,  ultimately,  the 
crimson  and  blue  entirely  vanished,  leaving  the 
still  strong  green  line  as  the  sole  representative  of 
the  radiations  of  hydrogen.  The  more  complicated 
spectrum  of  nitrogen  was,  under  similar  conditions, 
also  reduced  to  a  single  green  line ;  and,  as  might 
perhaps  have  been  expected,  a  mixture  of  hydrogen 
and  nitrogen  under  these  conditions  yielded,  when 
traversed  by  an  electric  discharge,  a  spectrum  con- 
sisting of  the  two  green  lines  already  observed. 
But,  by  now  moderating  the  discharge  so  that  the 
temperature  of  the  gases  should  be  reduced,  the 
green  nitrogen  line  in  its  turn  disappeared,  while 
that  due  to  hydrogen  still  shone  out  conspicuously; 
so  that,  in  a  mixture  known  to  contain  nitrogen, 


190          Recent  Advances  in  Astronomy. 

and  traversed  by  a  current  of  electricity  sufficient  in 
intensity  to  cause  a  gas  mixed  with  the  nitrogen 
to  glow,  no  trace  of  nitrogen  was  recorded  in  the 
spectrum. 

Changes  in  the  spectrum  of  calcium  are  no  less 
remarkable  or  important.  Compounds  of  the  metal 
calcium — the  metallic  base  of  lime — when  introduced 
into  the  flame  of  a  bunsen  burner  by  the  means 
already  described,  cause  the  flame  to  acquire  a  brick- 
red  tinge.  Observation  of  its  spectrum  shows  this 
light  to  be  composed  of  rays  of  different  colours, 
conspicuous  among  which  is  red — represented  in  the 
spectrum  by  a  broad  red  band.  When  introduced 
into  the  electric  arc,  the  temperature  of  which  is 
considerably  higher  than  that  of  the  bunsen  flame, 
the  vapour  of  calcium  gives  a  spectrum  in  which 
the  red  band  has  become  much  reduced,  while  a 
strong  blue  line,  invisible  in  the  flame  spectrum,  has 
appeared,  as  well  as  a  pair  of  brilliant  violet  lines. 
In  the  spark  from  the  induction-coil,  which  is  pro- 
bably at  a  still  higher  temperature,  the  spectrum 
entirely  loses  its  red  ray,  the  blue  becomes  fainter, 
while  the  violet  pair  are  far  more  strongly  developed 
than  before.  The  last  spectrum  of  calcium  is,  there- 
fore, as  different  from  the  first  as  if  two  different 
metals  had  been  subjected  to  examination.  Passing 
now  to  the  Sun,  we  find  among  the  Fraunhofer  lines 
the  reversed  images  of  the  lines  of  the  last — the 
spark  spectrum :  H  and  K,  the  great  dusky  pair  lying 
almost  at  the  limit  of  the  violet,  corresponding  in 
their  positions  with  the  two  broadened  violet  lines, 
and  a  fine  dark  line  to  which  no  special  name  has 


The  Analysis  of  Sunlight.  191 

been  given  with  the  blue  line.  It  is  further  interest- 
ing to  notice  that  in  light  condensed  from  solar  pro- 
minences— which  are  generally  regarded  as  hotter 
than  the  general  atmosphere — upon  the  slit  of  the 
spectroscope  (the  method  by  which  this  is  effected 
will  be  described  in  a  later  chapter),  the  blue  line 
has  in  its  turn  disappeared,  and  the  strong  pair, 
H  and  K,  alone  remain  as  the  representatives  of 
calcium. 

Sir  Norman  Lockyer  has  interpreted  the  change 
in  the  spectrum  of  calcium,  as  well  as  similar  in- 
stances presented  by  spectra  of  other  metals,  as  the 
direct  effect  of  increase  in  temperature,  and  main- 
tains that  they  lend  strong  support  to  the  view,  of 
which  he  has  made  himself  the  champion,  that  by 
increase  of  temperature  terrestrial  elements  become 
"  dissociated  "  or  resolved  into  still  more  elementary 
forms  of  matter.  According  to  this  view  the  pair  of 
violet  lines  are  not  radiated  by  the  vapour  of  calcium, 
but  by  the  vapour  of  some  element  contained  in  cal- 
cium and  dissociated  from  it  by  the  temperature  of 
the  electric  arc  and  that  of  the  atmosphere  of  the 
Sun,  while  the  substance  emitting  the  red  rays 
displayed  in  the  bunsen  burner  has  been  entirely 
decomposed  at  these  temperatures.  Similarly,  the 
substance,  the  radiation  of  which  contains  the  blue 
line,  is  first  dissociated  from  calcium  at  the  tempera- 
ture of  the  arc,  is  partially  dissipated  in  the  hotter 
spark,  and  is  entirely  destroyed  in  the  still  more 
intense  heat  of  the  prominences.  In  1897,  however, 
Sir  William  Huggins  succeeded  in  effecting  the 
same  changes  in  the  spectrum  of  calcium  by  reduc- 


192          Recent  Advances  in  Astronomy. 

ing  the  density  of  the  vapour,  without,  as  he  con- 
fidently believed,  the  change  being  accompanied  by 
any  appreciable  increase  in  temperature,  so  that  it 
appears  probable  that  in  the  experiments  first  de- 
scribed the  changes  appearing  on  increase  of  tem- 
perature may  have  been  only  indirectly  due  to  that 
cause,  the  direct  influence  having  been  the  reduction 
in  density  consequent  upon  the  expansion  of  the 
heated  vapours. 

Although  all  of  the  more  conspicuous  of  the 
Fraunhofer  lines  have  now  been  connected  with 
bright  lines  in  the  spectra  of  terrestrial  elements, 
the  great  majority  of  the  whole  are  still  unidentified. 
It  is,  of  course,  possible  that  many  or  even  all  of 
these  may  yet  be  found  to  correspond  with  the  bright 
lines  of  terrestrial  elements  if  it  should  become 
possible  to  produce  in  the  laboratory  conditions 
more  closely  approximating  to  those  that  exist  in 
the  atmosphere  of  the  Sun.  The  failure  to  detect 
the  faintest  trace  of  absorption  by  oxygen  in  the 
solar  radiations  is  very  remarkable,  in  connection 
with  the  extensive  distribution  and  supreme  impor- 
tance of  oxygen  in  the  atmosphere  and  in  the  crust 
of  the  Earth.  It  is  true  that  oxygen  is  magnificently 
represented  among  the  Fraunhofer  lines,  the  great 
groups  A  and  B  being  due  to  it,  but  there  is  no  doubt 
that  these  are  entirely  caused  by  absorption  in  the 
atmosphere  of  the  Earth.  They  become  decreasingly 
conspicuous  in  the  solar  spectrum  as  observations 
are  made  from  higher  and  higher  altitudes,  and  at 
such  a  rate  as  to  indicate  that  beyond  the  farthest 
limits  of  the  atmosphere  all  trace  of  them  would 


The  Analysis  of  Starlight.  193 

disappear  from  the  radiations  of  the  Sun.  It  is  of 
course  still  possible  that  oxygen  may  exist  in  the 
Sun's  atmosphere  under  physical  conditions  so 
differing  from  those  with  which  we  are  familiar 
that  its  spectrum  is  unrecognizable,  or  it  is  conceiv- 
able that  it  exists  dissociated  into  other  and  more 
elementary  forms  of  matter,  the  uninterpreted  record 
of  which  is  before  us  among  the  many  thousands  of 
the  Fraunhofer  lines  whose  language  has  still  to  be 
read. 


Chapter  V. 
The  Analysis  of  Starlight. 

In  the  previous  chapter  we  have  traced  the  suc- 
cession of  sure  though  laborious  steps,  by  which, 
from  the  decomposition  of  a  beam  of  sunlight  in 
Newton's  study,  a  new  science  has  been  constructed, 
that  has  given  us  a  revelation  of  the  chemistry  of  a 
body  nearly  a  hundred  million  miles  away  across 
apparently  empty  space,  the  very  suggestion  of 
which  would  have  seemed  utterly  preposterous  a 
hundred  years  ago.  That  a  beam  of  sunlight,  less 
than  a  thousandth  of  a  square  inch  in  section,  should 
contain  latent  within  it  the  record  of  the  constitution 
of  the  Sun's  atmosphere  may  well  induce  hesitation 
in  imagining  any  limit  to  the  powers  of  scientific 
methods.  While,  moreover,  steadily  extending 
astronomical  discovery  in  this  direction,  the  new 
method  has  been  developed  with  no  less  astounding 
success  along  other  lines.  It  was  considered  advis- 

(M520)  N 


194          Recent  Advances  in  Astronomy. 

able  to  ignore  these  for  the  time,  and  they  will 
form  the  subject  of  the  present  and  the  following 
chapter. 

We  have  seen  that  Fraunhofer  in  1814  directed  to 
the  light  of  the  stars  the  method  that  he  had  already 
applied  with  such  remarkable  results  to  that  of  the 
Sun ;  and  that  he  found  their  spectra  to  be  crossed, 
like  the  spectrum  of  the  Sun,  by  a  number  of  dark 
lines.  He  found,  moreover,  that  while  in  some 
cases  the  dark  lines  of  stellar  spectra  agreed  closely 
with  those  seen  in  the  spectrum  of  the  Sun,  they 
were  more  often  different  from  them  both  in  their 
positions  and  their  relative  intensity.  The  method 
adopted  by  Fraunhofer  in  these  investigations  was 
a  modification  of  that  applied  to  sunlight,  differing 
from  it  chiefly  in  that  it  did  not  involve  the  use  of 
a  slit,  and  has  since  been  generally  followed  in  all 
cases  in  which  it  has  not  been  essential  to  determine 
with  a  very  high  degree  of  accuracy  the  absolute — 
as  contrasted  with  the  relative — positions  of  the  dark 
lines.  A  telescope  is  directed  to  the  star,  and  the 
practically  parallel  rays  falling  upon  the  object-glass 
are  by  it  condensed  into  a  point-like  image  at  its 
principal  focus.  In  the  ordinary  use  of  the  telescope 
these  rays,  continuing  their  courses,  diverge  again 
after  meeting  at  the  focal  image,  and,  after  travers- 
ing the  eye-piece — a  system  of  lenses  equivalent  to 
a  magnifying-glass,  enter  the  eye.  It  is  convenient 
to  regard  the  eye  as  directly  observing  the  image  at 
the  focus  of  the  object-glass  by  the  aid  of  the  mag- 
nifying eye-piece.  If  now  a  prism  be  placed  in  the 
path  of  the  rays,  either  before  or  after  they  enter  the 


The  Analysis  of  Starlight.  195 

telescope,  since  different  colours  are  deflected  dif 
ferently,  the  point-like  focal  image  becomes  expandeo 
into  a  line  of  coloured  light.  Such  a  line  is,  how- 
ever, obviously  inconvenient  for  examination;  but 
by  further  interposing  a  " cylindrical  lens" — a  lens 
having  for  its  faces  portions  of  cylindrical  instead  oi 
spherical  surfaces — anywhere  in  the  path  of  the  rays, 
the  line  becomes  broadened  out  into  a  band  of  de- 
finite width,  in  which  dark  lines  are  clearly  visible. 
The  line  of  light  necessary  for  the  purpose  of  an- 
alysis, instead  of  being  obtained  by  a  slit,  is  formed 
by  the  extension  of  the  point-like  image  of  the  star 
into  a  line  by  the  cylindrical  lens.  In  Fraunhofer's 
arrangement  of  apparatus,  the  rays  passed  through 
the  prism  immediately  before  entering  the  telescope, 
and  the  cylindrical  lens  was  so  adjusted  as  to  be 
just  beyond  the  special  line  of  light  formed  at  the 
principal  focus. 

No  observations  sufficiently  delicate  to  add  any- 
thing material  to  Fraunhofer's  discoveries  in  rela- 
tion to  stellar  spectra  were  made  before  the 
publication  of  KirchhofFs  researches  into  the  origin 
of  the  Fraunhofer  lines  in  the  solar  spectrum.  By 
these  researches,  the  dark  lines  in  the  spectrum  of 
the  Sun  were  traced  beyond  doubt  to  the  absorption 
of  the  colours  corresponding  to  them  in  a  solar 
atmosphere,  and  there  could  be  little  hesitation  in 
extending  the  same  principle  to  the  explanation  of 
the  similar  lines  in  the  spectra  of  stars.  The  sun- 
like  character  of  the  stars,  already  apparent  with 
respect  to  their  total  luminosity  from  the  establish- 
ment of  the  Copernican  system,  became  more 


196          Recent  Advances  in  Astronomy. 

intimately  confirmed  by  the  evidence  revealed  in 
the  analysis  of  their  light  that,  like  the  Sun,  their 
glowing  photospheres  were  enveloped  in  absorbent, 
and  therefore  cooler,  atmospheres. 

Although  observations  of  the  spectra  of  stars  had 
been  made  for  a  few  years  previously  by  Father 
Secchi  at  Rome,  their  examination  was  first  attacked 
systematically  by  Sir  William  Huggins  and  Dr. 
Miller  about  the  year  1863.  At  that  time,  and  even 
at  the  present,  the  visual  observation  of  all  but  the 
most  brilliant  stellar  spectra  was  a  most  trying  and 
delicate  task.  The  faint  light  of  a  star,  even  when 
collected  over  the  extended  area  of  a  large  object- 
glass  and  condensed  to  its  focal  point,  is  immeasur- 
ably feeble  when  compared  with  sunlight.  It  is 
necessary  to  further  enfeeble  it;  first,  by  extending 
it  into  a  spectral  line,  and,  secondly,  by  expanding 
this  line  into  a  band;  while,  from  the  atmospheric 
unsteadiness,  of  which  the  familiar  appearance  of 
twinkling  is  a  result,  the  excessively  faint  and,  in 
most  cases,  barely  visible  spectrum  is,  together  with 
its  delicate  system  of  lines,  thrown  into  a  continual 
state  of  tremor.  In  spite  of  such  difficulties,  how- 
ever, Huggins  and  Miller  succeeded  in  detecting  in 
the  spectra  of  several  stars  lines  corresponding  with 
those  in  the  spectrum  of  the  Sun  as  well  as  with 
many  bright  lines  in  the  spectra  of  glowing  vapours 
of  terrestrial  elements.  They  also  thoroughly  con- 
firmed Fraunhofer's  observations  that  in  many  cases 
the  spectra  of  stars  were  strikingly  different  in  the 
arrangement  and  intensity  of  their  dark  lines  from 
that  of  the  Sun. 


The  Analysis  of  Starlight.  197 

While  engaged  in  these  observations,  Sir  William 
Huggins  applied  the  spectroscope  to  the  investiga- 
tion of  the  physical  condition  of  the  nebulae.  We 
have  traced  in  an  earlier  chapter  the  development 
of  astronomical  discovery  and  thought  with  refer- 
ence to  these  cosmic  clouds.  We  have  seen  that,  at 
the  time  of  Sir  William  Huggins's  observation,  the 
view  was  very  generally  entertained  that  they  were 
stellar  systems,  the  constituent  stars  being,  in  the 
great  majority  of  cases,  too  faint  to  be  individually 
distinguishable,  though  the  great  telescope  of  the 
Earl  of  Rosse  appeared  to  have  recently  effected 
the  resolution  of  some  of  the  nobler  examples.  We 
have  also  followed  the  main  features  of  Sir  William 
Huggins's  discovery.  The  telescope  was  directed 
to  a  small  but  rather  bright  nebula  in  the  constella- 
tion of  the  Dragon.  The  image  of  the  nebula, 
formed  by  the  condensation  of  its  rays  by  the  object- 
glass,  was  no  longer  a  point,  as  with  a  star,  but  an 
assemblage  of  points,  one  corresponding  to  each  of 
the  luminous  points  of  the  nebula ;  in  fact,  an  image 
of  the  nebula,  such  that,  if  a  screen  or  photographic 
plate  had  been  placed  behind  the  object-glass,  at  a 
distance  from  it  equal  to  its  focal  length,  a  perfect, 
though  excessively  faint,  picture  of  the  nebula 
would  have  been  formed  upon  it.  As  a  cylindrical 
lens  would  not  have  expanded  such  an  image  into 
a  line,  it  was  considered  expedient  to  make  use  of 
the  more  usual  slit.  The  eye-piece  of  the  telescope 
was  removed,  and  a  spectroscope  was  fitted  in  its 
place,  the  length  of  the  collimator  lying  along  the 
main  axis  of  the  telescope,  while  the  slit  was  ad- 


198          Recent  Advances  in  Astronomy. 

justed  to  the  position  of  the  principal  focus  of  the 
object-glass.  In  this  position  the  image  of  the 
nebula  was  formed  upon  the  "  slit-plate" — the  pair 
of  metal  plates,  by  the  separation  of  which  the  slit 
was  produced.  A  thin  slice  of  the  nebula  light  thus 
entered  the  slit,  and  was  subjected  to  analysis  and 
subsequent  examination  in  the  ordinary  way. 

At  a  first  glance  the  spectrum  of  the  nebula 
appeared  to  be  entirely  monochromatic,  its  light 
being  all  condensed  in  a  single  green  line.  Closer 
examination,  however,  revealed  the  presence  of  a  far 
fainter  line  rather  higher  up  the  spectrum — that  is, 
toward  the  blue — as  well  as  a  third,  exceedingly 
faint,  and  still  higher  in  the  spectrum.  The  failure 
up  to  that  time  to  observe  a  spectrum  of  isolated 
bright  lines  otherwise  than  from  the  radiations  of  a 
glowing  gas,  had  caused  such  a  spectrum  to  be  re- 
garded as  a  crucial  test  of  gaseous  constitution — a 
conclusion  thoroughly  supported  by  all  later  spectro- 
scopic  work — and  the  observation  was  therefore 
universally  accepted  as  demonstrating  the  gaseous 
constitution  of  the  nebula.  Of  the  three  lines  ob- 
served in  the  spectrum,  the  third  coincided  in  posi- 
tion with  the  green  line  of  hydrogen,  the  persistent 
character  of  which  was  so  strikingly  illustrated  by 
Sir  Edward  Frankland  and  Sir  Norman  Lockyer 
five  years  later ;  neither  of  the  other  two  probably 
correspond  with  any  lines  that  have  been  obtained 
in  terrestrial  experiments,  though,  at  the  time,  the 
first  was  thought  to  occupy  the  position  of  a  line  of 
nitrogen,  to  which,  however,  later  measurements 
have  shown  it  to  be  only  exceedingly  close. 


The  Analysis  of  Starlight.  199 

Of  the  many  nebulae  that  have  been  subjected 
to  spectroscopic  examination  since  the  date  of  Sir 
William  Huggins's  first  observation,  about  one- 
half  have  been  found  to  yield  a  spectrum  of  bright 
lines.  It  may  be  confidently  asserted  that  the 
incandescent  matter  of  all  these  is  gaseous.  With 
a  few  exceptions,  the  remainder  yield  faint  continu- 
ous spectra  unmarked  by  any  evidence  of  special 
radiation  or  absorption,  but  it  is  not  possible  to 
infer  from  this  alone  that  they  are  not  either  partially 
or  entirely  gaseous.  Although  a  gas  alone  appears 
to  possess  the  power  of  giving  rise  to  a  spectrum 
of  bright  lines,  yet,  under  not  abnormal  conditions, 
its  light  may  yield  a  continuous  spectrum  indis- 
tinguishable from  that  of  an  incandescent  solid  or 
liquid  body.  Excessive  pressure  and  great  depth 
of  the  radiating  gas  tend  to  bring  about  such  a 
result,  but  a  continuous  spectrum  has  been  obtained 
from  a  small  quantity  of  oxygen  contained  in  a 
glass  tube  under  considerably  less  than  the  at- 
mospheric pressure.  It  is  not  at  present  possible 
to  interpret  a  continuous  nebular  spectrum. 

The  first  and  brightest  of  the  nebular  lines 
detected  by  Sir  William  Kuggins  appears  to  be 
specially  characteristic  of  bright-line  nebulae.  In 
every  one  of  their  spectra  it  appears  as  the  brightest 
line  of  the  series,  while  in  many  it  occurs  as  the 
sole  representative,  other  lines,  though  probably 
present,  being  too  faint  for  detection.  In  a  few 
instances  other  lines  than  the  three  originally  seen 
have  been  observed,  nearly  thirty  having  been 
detected  by  visual  and  photographic  observations 


200          Recent  Advances  in  Astronomy. 

in  that  of  the  Great  Nebula  of  Orion.  Of  these, 
hydrogen  lines,  including  the  crimson  c,  and  a 
yellow  line  due  to  the  element  helium,  the  signi- 
ficance of  which  will  be  seen  later,  are  the  only 
ones  that  have  been  reproduced  in  laboratory 
experiments,  so  that  the  chemistry  of  the  nebulae 
has  only  so  far  been  connected  with  that  of  the 
earth  through  the  elements  hydrogen  and  helium. 

In  its  first  application  the  spectroscope  had  been 
essentially  an  instrument  of  chemical  research.  In 
its  demonstration  of  the  physical  condition  of 
nebulae  it  had  been  brought  to  bear  upon  problems 
possessing  a  physical,  as  well  as  a  purely  chemical, 
interest;  but  it  was  now  to  invade  the  domain  of 
physical  science,  pure  and  simple,  and  with  the 
most  remarkable  and  far-reaching  results. 

In  1848  Christian  Doppler  of  Prague  had  directed 
attention  to  the  fact  that  the  apparent  pitch  of  a 
musical  note  became  affected  during  any  variation 
in  the  distance  separating  the  instrument  emitting 
it  from  the  ear.  A  note  appears  to  rise  in  pitch 
as  the  source  of  sound  approaches,  and  to  fall  in 
pitch  as  it  recedes  from  the  ear.  The  effect  was 
recognized  as  a  natural  consequence  of  the  wave 
transmission  of  sound,  and  Doppler  showed,  that, 
if  light  were  also  transmitted  by  wave  motion,  it 
should  follow  from  analogous  reasoning  that  the 
colour  of  an  object  should  be  affected  by  the  motion 
of  the  source,  becoming  more  violet  as  the  object 
approached,  and  inclining  toward  red  as  it  receded 
from,  the  observer. 

It  is  a  well-known  fact  that  the  sensation  of  sound 


The  Analysis  of  Starlight.  201 

is  due  to  the  transmission  of  vibrations  from  a 
sounding  body  to  the  ear  through  the  agency  of 
wave  motion  in  the  air;  and  that  the  pitch  of  a 
note  is  the  result  of  the  frequency  of  the  vibrations 
—  the  number  executed  in  a  second  of  time  —  a 
doubling  of  frequency  causing  a  rise  in  pitch 
recognized  by  the  ear  as  an  octave.  At  each 
vibration  of  the  sounding  body  a  single  wave  is 
generated  in  the  immediately  surrounding  air;  the 
wave  expands  outward  in  an  ever -increasing 
spherical  surface — as  a  ripple  on  the  surface  of  a 
pool  unruffled  by  wind  extends  in  a  continually 
expanding  circle — and,  upon  arriving  at  the  ear, 
imparts  to  the  auditory  apparatus  a  vibration 
similar  to  that  by  which  it  originated.  The  vibra- 
tion of  the  sounding  body  continuing,  waves  are 
continually  generated,  and  follow  in  regular  succes- 
sion, so  that,  under  normal  conditions,  as  many 
enter  the  ear  every  second  as  are  generated  by  the 
sounding  body.  The  frequency  of  the  note  heard 
is  therefore  that  of  the  source  of  sound.  If,  how- 
ever, the  source  is  approaching  the  ear,  this  corre- 
spondence is  no  longer  maintained.  The  source 
generates  waves  with  the  same  rapidity  as  before, 
a  single  wave  being  produced  by  each  vibration; 
but  it  is  important  to  notice  that  the  speed  with 
which  the  waves  travel  through  the  air — that  is, 
the  velocity  of  sound — is  the  same  as  when  the 
source  was  at  rest,  since  it  may  be  shown  from 
mechanical  principles  that  the  velocity  of  wave 
motion  is  determined  solely  by  the  physical  pro- 
perties of  the  medium  in  which  they  exist,  and  is 


202          Recent  Advances  in  Astronomy. 

entirely  independent  of  any  motion  of  the  source. 
Since,  therefore,  the  waves  are  travelling  through 
the  air  with  the  same  speed  as  when  the  source 
was  at  rest,  and  the  source  is  now  following  them, 
they  will  be  crowded  together,  and  the  length  of 
each  will  be  decreased.  The  waves,  being  shorter 
than  before,  and  still  forming  a  continuous  series 
travelling  with  the  original  speed,  will  enter  the 
ear  in  more  rapid  succession,  the  frequency  of 
vibration  of  the  auditory  apparatus  will  increase, 
and  the  note  will  rise  in  pitch.  From  analogous 
reasoning  it  will  be  seen  without  difficulty  that  a 
recession  of  the  source  will  result  in  a  diminution 
of  frequency  of  vibration,  and  consequently  in  a 
fall  of  pitch. 

It  may  be  well  to  give  a  further  and  more  detailed 
illustration  of  this  very  important  principle.  Let 
us  imagine  a  tuning-fork  at  rest,  and  radiating 
waves  in  the  air,  each  i  foot  in  length  —  waves 
that  would  correspond  to  a  note  about  two  octaves 
above  the  middle  C  of  the  piano.  By  the  time  ten 
vibrations  had  occurred,  and  ten  waves  had  conse- 
quently been  generated,  the  first  wave  would  have 
travelled  10  feet  from  the  fork,  since  the  whole 
ten,  each  a  foot  in  length,  would  form  a  continuous 
series.  Now  imagine  the  fork  to  move  forward, 
and  assume  its  velocity  to  be  one-fifth  that  of  the 
waves — that  is,  one-fifth  the  speed  of  sound — and 
under  these  new  conditions  imagine  the  original 
ten  vibrations  to  be  repeated.  As  before,  ten 
waves  will  be  generated,  each  following  the  last 
in  regular  succession.  At  the  instant  of  generation 


The  Analysis  of  Starlight.  203 

of  the  tenth,  the  first  wave  will  have  reached  the 
same  point  as  before,  its  speed  being  unaffected 
by  the  motion  of  the  fork ;  but,  during  its  progress, 
the  fork  has  been  following  it  with  a  speed  one- 
fifth  of  its  own,  so  that  the  distance  separating  it 
from  the  fork  is  four-fifths  of  its  former  value.  As 
there  are  the  same  number  of  waves  lying  between 
it  and  the  fork,  each  must  therefore  be  four-fifths 
as  long  as  originally.  The  shorter  waves,  travel- 
ling with  the  same  speed  as  before,  enter  the  ear 
in  more  rapid  succession,  'and  the  frequency  of 
vibration  is  increased  to  five-fourths  of  its  former 
value.  In  every  case  the  change  in  frequency  can 
be  calculated  in  this  simple  manner  from  the  relation 
between  the  speed  of  the  moving  source  and  that 
of  the  waves. 

It  is  quite  easy  to  notice  the  fall  in  pitch  in  the 
note  of  the  whistle  of  an  engine  when  passing  the 
observer  at  express  speed.  For  a  speed  of  60  miles 
an  hour  (88  feet  per  second),  the  speed  of  sound 
being  noo  feet  per  second,  the  reader  should  find 
little  difficulty  in  showing  that  the  frequency  of  the 
whistle  is  raised  to  i  "087  of  its  normal  value  while 
approaching,  and  decreased  to  -926  of  it  while 
receding.  The  relative  frequencies  of  1*087  and 
•926  correspond  to  an  interval  of  nearly  three  semi- 
tones, that  from  do  to  lat  in  music,  and  such  a  change, 
which  occurs  at  the  instant  that  the  engine  passes 
the  observer,  can  scarcely  escape  the  notice  of  the 
least  musical  ear.  The  change  is,  of  course,  still 
more  strongly  marked  when  the  observer,  instead  of 
being  at  rest,  is  travelling  at  express  speed  in  the 


204          Recent  Advances  in  Astronomy. 

direction  opposite  to  that  of  the  whistling  engine. 
The  fall  in  pitch  of  the  bell  of  a  passing  bicycle  is 
quite  appreciable  to  anyone  with  a  fairly  sensitive 
ear,  even  when  at  rest  by  the  side  of  the  road. 

It  is  clear  that  if  light  depends  upon  the  trans- 
mission of  waves  in  the  ether,  similar  changes 
must  be  produced  in  those  waves  by  the  motion 
of  the  source  of  light  towards  or  from  the  observer. 
By  a  motion  of  approach,  the  waves  entering  the 
eye  must  be  reduced  in  length;  they  must  arrive 
in  more  rapid  succession,  and  a  colour  more  inclin- 
ing to  violet  must  result;  while,  from  a  motion  of 
recession,  the  waves  must  be  drawn  out;  they  must 
enter  the  eye  in  less  rapid  succession,  and  the 
colour  must  appear  lower  in  the  spectral  series. 
Such  was  the  prediction  of  Doppler,  and  he  sug- 
gested that  the  strongly-marked  colours  of  certain 
stars  might  originate  in  their  rapid  motions. 

At  the  time  of  the  enunciation  of  this  principle, 
the  most  serious  objection  to  its  suggested  applica- 
tion appeared  to  lie  in  the  excessive  speeds  with 
which  it  was  necessary  to  suppose  the  stars  to  be 
endued.  By  the  rush  of  a  star  towards  the  Earth, 
the  whole  series  of  spectral  colours  present  in  its 
radiations  were  supposed  to  move  up  the  spectrum, 
so  that  the  light  received  appeared  to  be  abundantly 
rich  in  violet  and  poor  in  red  rays,  the  intermediate 
colours  appearing  the  same  as  before,  since,  as 
each  became  displaced  towards  the  violet,  its  place 
would  be  supplied  by  the  similarly  affected  rays 
immediately  below  it.  Owing  to  the  high  velocity 
of  light,  it  was  clear,  that,  to  effect  any  appreciable 


The  Analysis  of  Starlight.  205 

change  of  colour  by  such  a  process,  velocities  of 
many  thousands  of  miles  per  second  must  be 
imagined  among  the  stars,  and  there  appeared  to 
be  no  warrant  for  so  extravagant  an  assumption. 

A  still  more  fatal  objection  to  Doppler's  theory 
became  apparent,  when,  in  the  years  immediately 
following,  the  extension  of  the  spectrum  into  its 
invisible  ultra-violet  and  infra-red  regions  was 
discovered.  From  the  existence  of  these  invisible 
radiations,  it  would  follow  that  the  colours  of  the 
visible  spectrum  of  a  star  should  be  unchanged 
by  its  approach  or  recession.  A  motion  of  ap- 
proach would  cause  the  frequency  of  each  radiation 
to  increase,  and,  in  consequence,  each  colour  would 
become  more  violet  in  hue,  and  would  experience 
greater  deviation  by  the  prism.  Each  colour  would, 
therefore,  be  displaced  towards  the  violet  end  of 
the  spectrum,  assuming  the  previous  tint,  while 
occupying  the  former  position,  of  the  colour  im- 
mediately before  it.  The  lowest  red  rays  would 
move  farther  into  the  spectrum,  their  colour  at  the 
same  time  becoming  brighter;  but  the  frequency 
of  the  rays  immediately  below  them,  previously 
just  too  low  to  excite  the  sensation  of  vision,  would 
be  so  increased  that  they  would  appear  in  the 
spectrum  just  within  its  lowest  limits,  and  would 
take  the  place  of  those  deep  rays  that  had  been 
displaced  upwards.  Similarly,  the  frequency  of 
the  extreme  violet  waves  would  be  so  increased 
that  they  would  enter  the  invisible  region  beyond. 
The  colours  of  the  visible  spectrum  would  there- 
fore be  the  same  as  when  the  star  was  at  rest. 


206          Recent  Advances  in  Astronomy. 

There  is  no  doubt  that  the  colour  of  a  star  is  a 
physical  fact,  initially  impressed  upon  its  radiations, 
and  that  it  cannot  be  explained  by  any  theory  of 
optical  illusion. 

So  far  the  attempt  to  apply  Doppler's  principle 
to  physical  astronomy  had  been  attended  with 
failure;  but  in  1848  Fizeau  indicated  a  method  by 
which  it  might  still  be  possible  to  detect  by  its  aid 
evidence  of  the  approach  or  recession  of  stars  in 
their  analysed  light.  It  was  suggested  that,  instead 
of  attempting  to  detect  the  evidence  of  motion  in 
the  entire  light  of  stars,  close  attention  should  be 
directed  to  the  exact  positions  of  the  dark  lines  by 
which  their  spectra  were  crossed.  We  have  seen 
that  a  dark  line  in  a  spectrum  indicates  the  position 
in  it  of  colour  absent  from  the  radiations;  and  it 
follows  that,  by  a  motion  of  approach  of  a  star, 
since  the  colours  that  are  just  more  and  just  less 
refrangible  than  the  missing  one  will  be  equally 
displaced  up  the  spectrum,  the  gap  separating 
them  will  be  similarly  displaced ;  in  other  words, 
every  dark  line  in  the  spectrum  of  an  approaching 
star  should  be  displaced  toward  the  violet  of  the 
spectrum,  while,  from  analogous  reasoning,  every 
dark  line  in  the  spectrum  of  a  receding  star  should 
be  displaced  toward  the  red. 

After  several  unsuccessful  experiments,  Sir 
William  Huggins  felt  justified  in  announcing  in 
1868  that  he  had  succeeded  in  detecting  in  the 
spectrum  of  Sirius  such  a  displacement  of  one  of 
its  spectral  lines  as  would  result  from  a  motion  of 
recession  of  the  star.  The  selection  of  Sirius  for 


The  Analysis  of  Starlight.  207 

the  purpose  of  the  research  was  due  to  the  great 
intensity  of  its  light,  as  well  as  from  the  strongly- 
marked  character  and  undoubted  origin  of  its 
spectral  lines.  It  has  been  found,  however,  more 
recently,  that  these  advantages  are  seriously  dis- 
counted by  the  grave  disadvantage  arising  from 
the  ill-defined  character  of  the  lines.  The  visual 
spectrum  of  Sirius  differs  from  that  of  the  Sun  in 
the  extraordinary  emphasis  of  the  dark  lines  of 
hydrogen  absorption — indeed,  under  ordinary  con- 
ditions these  are  all  that  are  visible,  though,  when 
the  atmosphere  is  steady  and  an  instrument  of  high 
optical  quality  is  employed,  a  great  number  of  fine 
dark  lines,  many  of  them  corresponding  with 
bright  lines  in  the  spectrum  of  iron,  may  also  be 
distinguished.  The  dark  hydrogen  lines  in  the 
spectrum  of  Sirius  are  about  six  times  as  broad  as 
those  in  the  solar  spectrum,  and,  unlike  the  latter, 
which  are  clearly  marked  and  sharply  defined, 
those  of  the  star  are  hazy  and  pass  by  insensible 
degrees  into  the  bordering  light  of  the  spectrum. 
There  is  good  reason  to  regard  this  difference  as 
indicating  a  greater  density  of  the  hydrogen  in  the 
atmosphere  of  the  star;  since,  while  at  a  low 
density,  as  in  the  ordinary  vacuum-tube,  glowing 
hydrogen  gives  a  spectrum  consisting  of  bright 
lines  that  resemble  the  dark  lines  in  the  spectrum 
of  the  Sun  in  being  fine  and  sharp,  with  increased 
density  of  the  gas  the  lines  become  broad  and 
badly  defined  at  their  edges ;  and  when  the  gas  is 
under  a  pressure  approaching,  though  still  distinctly 
below,  that  of  the  atmosphere,  its  bright  lines  very 


2o8          Recent  Advances  in  Astronomy. 

closely  resemble  in  their  definition  -the  dark  lines  of 
the  Sirian  spectrum. 

To  determine  whether  a  dark  line  in  the  spectrum 
of  a  star  coincides  exactly  with  a  corresponding 
bright  line  in  the  spectrum  of  an  incandescent 
terrestrial  vapour,  it  is  absolutely  necessary  that 
both  should  appear  in  the  field  of  view  at  the  same 
time,  and  this  condition  necessitates  the  rejection  of 
the  more  convenient  cylindrical  lens  in  favour  of 
the  slit.  In  Huggins's  experiment  the  spectro- 
scope was  so  adjusted  that  the  slit  was  very  near 
but  not  coincident  with  the  principal  focus  of  the 
object-glass  of  a  telescope  of  8  inches  of  aperture, 
so  that  a  small  length  of  it  became  illuminated  by 
the  nearly  condensed  rays  of  the  star,  and  a  spec- 
trum of  a  corresponding  width  was  produced.  At 
the  same  time,  rarefied  hydrogen,  contained  in  a 
glass  tube  placed  just  beyond  the  object-glass,  was 
made  to  glow  by  the  electric  discharge  from  an  in- 
duction-coil. Under  these  conditions  the  narrow 
spectrum  of  the  star  became  visible,  while  extending 
right  across  it  were  the  bright  lines  of  the  glowing 
hydrogen.  Attention  was  specially  directed  to  the 
most  conspicuous  of  the  dark  lines — that  in  the 
green  part  of  the  spectrum,  the  far  more  delicate 
bright-green  line  of  the  glowing  gas  appearing  to 
traverse  it  in  the  direction  of  its  length.  The  most 
careful  observations  of  the  lines  were  made  many 
times,  and  Huggins  felt  confident  that  the  bright 
line,  though  it  appeared  projected  upon  the  dark 
one,  did  not  lie  along  the  middle  of  it,  but  was 
placed  rather  lower  down  toward  the  red  of  the 


The  Analysis  of  Starlight.  209 

spectrum.  Since,  however,  it  was  quite  conceivable 
that  expansion  of  the  dark  line  in  the  spectrum  of 
the  star,  presumably  due  to  increased  density  of  the 
absorbing  gas,  might  not  have  taken  place  equally 
upon  both  sides,  the  observation  was  not  quite 
conclusive,  but  Huggins  carefully  examined  the 
spectrum  of  glowing  hydrogen  at  varying  densities, 
and  found  that  the  broadening  accompanying 
increase  of  density  was  in  every  case  entirely 
symmetrical  upon  either  side.  The  want  of  co- 
incidence between  the  centres  of  the  lines  was 
therefore  confidently  attributed  to  motion  in  the 
line  of  sight,  and,  from  its  extent,  after  allowing  for 
the  motion  of  the  Earth  in  its  orbit  at  the  time, 
Huggins  estimated  that  the  star  was  receding  from 
the  Sun  at  the  rate  of  29^  miles  per  second. 

In  the  following  year  Huggins  extended  his 
observations  to  other  stars,  and  was  successful  in 
detecting  displacements  of  spectral  lines  in  thirty 
instances.  With  some  stars,  such  as  Rigel  and 
Castor,  the  hydrogen  lines  were  displaced  towards 
the  red  end  of  the  spectrum,  an  indication  of  reces- 
sion; with  others,  including  Arcturus  and  Vega, 
they  were  raised  towards  the  violet,  and  denoted 
approach.  Further,  with  the  view  of  confirming 
beyond  doubt  both  the  soundness  of  the  principle 
and  the  possibility  of  its  practical  application,  the 
spectrum  of  Venus  was  observed  at  times  when, 
from  the  position  of  the  planet  in  its  orbit,  its 
motion  was  known  to  be  directed  towards  and  away 
from  the  Earth.  Since  the  light  of  Venus  is  re- 
flected sunlight,  the  ordinary  Fraunhofer  lines  are 

(M520)  O 


210          Recent  Advances  in  Astronomy. 

represented  in  its  spectrum,  but  a  careful  examina- 
tion of  the  selected  hydrogen  lines  showed,  as  had 
been  confidently  anticipated,  displacements  from 
the  positions  of  the  lines  of  terrestrial  hydrogen, 
and  precisely  such  displacements  as  were  demanded 
by  theory  from  the  speed  of  the  planet  relatively  to 
that  of  light. 

The  approach  or  recession  of  an  object  is  known 
technically  as  its  motion  in  the  line  of  sight.  It  is 
clearly  but  one  component  of  the  whole  movement, 
the  other  being  a  drift  at  right  angles  to  or  athwart 
the  line  of  sight,  and  the  determination  of  the  com- 
plete motion  demands  a  knowledge  of  both  of  the 
components.  Motion  of  a  star  across  the  line  of 
sight  is  indicated  in  its  so-called  "  proper  motion  ", 
or  apparent  rapidity  of  drift  across  the  face  of  the 
sky,  but,  since  the  apparent  drift  represented  by  a 
given  velocity  obviously  depends  inversely  upon 
the  distance  of  the  star,  it  is  essential  to  know  the 
distance  before  translating  proper  motion  into 
definite  velocity.  Our  present  knowledge  of  the 
distances  of  stars  is  so  imperfect  that  only  in  a  very 
few  instances  is  it  possible  to  make  the  application 
with  any  approach  to  accuracy,  but  in  a  few  in- 
stances some  rough  approximation  has  probably 
been  possible.  The  unique  power  of  the  spectro- 
scopic  method  of  determining  motion  lies  in  the  fact 
that  the  exact  interpretation  of  its  record  is  entirely 
independent  of  the  distance  of  a  star,  the  same 
displacement  of  spectral  lines  resulting  from  a 
definite  movement  in  the  line  of  sight,  whether  the 
luminous  body  is  a  member  of  the  Solar  System, 


The  Analysis  of  Starlight.  211 

or  whether  it  lies  at  the  extreme  limits  of  fathomable 
space. 

There  can  assuredly  be  but  little  hesitation  in 
placing  the  detection  and  measurement  of  the 
motions  of  stars  in  the  line  of  sight  as  among  the 
greatest  achievements  of  physical  science.  Before 
the  statement  of  Doppler's  principle,  the  mere  detec- 
tion of  such  movements  must  have  appeared  to  be 
beyond  the  very  possibility  of  human  endeavour,  at 
any  rate  without  observations  extending  over  many 
thousands  of  years.  From  the  motion  of  a  star  in 
line  of  sight,  its  position  upon  the  face  of  the 
heavens  is  unaffected.  It  is  true  that  by  the  con- 
tinuance of  such  movement,  in  the  course  of  time 
a  change  in  the  apparent  brightness  of  a  star  would 
result,  as  also  an  alteration  of  its  parallax;  but  so 
many  thousands  of  years  must  elapse  before  either 
would  become  appreciable  to  the  most  refined 
observation,  that  but  little  enthusiasm  could  be 
aroused  by  the  contemplation  of  the  ultimate  possi- 
bility of  the  successful  application  of  either  method. 
By  the  discovery  of  Huggins,  however,  an  observa- 
tion, demanding,  it  is  true,  the  utmost  delicacy,  but 
which  need  not  extend  over  more  than  a  few 
minutes,  has  proved  sufficient.  Again,  few  dis- 
coveries have  furnished  a  finer  illustration  of  the 
debt  that  one  science  so  frequently  owes  to  its 
sisters,  and  of  the  unexpected  nature  of  the  con- 
junction towards  which  accumulation  of  knowledge 
is  tending.  Upon  one  line  we  see  the  story  of 
sunlight  first  roughly  sketched  in  the  ray  dispersed 
by  Newton's  prism;  told  with  more  detail  in  the 


212          Recent  Advances  in  Astronomy. 

improved  conditions  of  experiment  devised  by 
Wollaston  and  Fraunhofer;  and  again  with  a  still 
fuller  meaning  in  the  shrewd  conjectures  of  Stokes 
and  in  the  experiments  of  Kirchhoff.  Upon  a 
converging  line  we  trace  the  first  conception  of  the 
wave  theory  of  light  in  the  genius  of  Huygens, 
and,  after  a  century  of  neglect,  its  restoration  and 
establishment  upon  a  firm  foundation  by  Young. 
Anon  comes  Doppler  still  discovering,  though 
mistaken  as  to  the  exact  course  he  was  directing; 
then  the  direction  of  the  course  towards  its  proper 
goal  by  Fizeau;  and,  later,  its  magnificent  attain- 
ment in  the  experimental  skill  of  Sir  William 
Huggins. 

In  1870,  two  years  after  Huggins's  discovery,  Dr. 
Hermann  Vogel,  who  then  had  charge  of  a  private 
observatory  at  Bothcamp,  was  attracted  to  the 
measurement  of  stellar  motions  in  the  line  of  sight. 
For  four  years  at  Bothcamp,  and  for  a  further 
period  of  thirteen  years  at  Potsdam,  where  he  be- 
came possessed  of  instrumental  means  of  greater 
power,  Vogel  carried  out  measurements  upon 
practically  the  same  method  as  that  originally 
adopted  by  Huggins.  In  1887,  however,  by  which 
time  the  photographic  gelatine  dry  plate — first  in- 
troduced by  Mr.  Kennet  in  1876 — had  reached  a 
high  degree  of  perfection,  and  had  been  introduced 
with  remarkable  success  in  other  branches  of 
astronomy,  Vogel  applied  it  to  the  purpose  of  his 
work,  and  soon  became  convinced,  that,  by  photo- 
graphing the  spectrum  of  a  star  together  with  the 
bright  lines  of  glowing  hydrogen  or  some  other 


The  Analysis  of  Starlight.  213 

terrestrial  vapour  introduced  for  the  purpose  of 
exact  comparison,  and  by  subjecting  the  com- 
pound spectrum  so  photographed  to  microscopic 
examination,  a  far  higher  degree  of  accuracy  could 
be  attained  than  was  possible  in  visual  observation. 
From  that  date  to  the  year  1891  the  photographic 
method  was  consistently  followed  at  Potsdam,  and 
from  the  consistency  between  independent  observa- 
tions of  the  same  star  at  different  times  there  can  be 
no  hesitation  in  regarding  the  results  as  constituting 
the  most  exact  record  that  has  so  far  been  acquired 
of  the  motions  of  stars  in  the  line  of  sight.  The 
speeds  of  approach  and  recession  of  the  following 
eight  more  familiar  stars  are  taken  from  Vogel's 
results,  the  velocities  being  given  in  miles  per 
second.  Due  allowance  has  been  made  in  every 
instance  for  the  direction  of  the  speed  of  the  Earth 
in  its  orbit — 18*7  miles  per  second — at  the  time  of 
observation,  so  that  the  numbers  are  actually  the 
velocities  of  the  stars  relatively  to  the  Sun.  So  far, 
no  star  has  been  found  to  possess  a  higher  velocity 
in  the  line  of  sight  than  Aldebaran. 

VELOCITIES  OF  APPROACH  AND  RECESSION  OF  STARS 
(Miles  per  second). 

Approaching  Stars.  Receding  Stars. 

Sirius,           ...         ...  9'8  J     Aldebaran,    ...         ...  30*2 

The  Pole  Star,         ...  i6'i          Rigel, 10*2 

Arcturus,       ...         ...  4*8  \     Capella,          ...         ...  15*2 

Vega,             9-5  ,     Betelgeux,      107 

It  was  during  the  course  of  these  observations 
that  the  last  step  was  taken  in  the  demonstration  of 
the  existence  of  the  dark  companion  of  Algol.  The 


214         Recent  Advances  in  Astronomy. 

essential  principles — other  than  the'  spectroscopic 
ones — that  underlie  the  investigation  have  already 
been  given  in  an  earlier  chapter,1  and  the  reader 
will  now  have  no  difficulty  in  completing  the  story 
of  the  discovery.  It  had  been  shown  to  follow  from 
the  laws  of  motion  that,  if  the  regular  and  con- 
tinually repeated  fading  observed  in  the  light  of 
Algol  were  due,  as  was  strongly  suspected,  to  its 
periodic  eclipse  by  a  dark  star  revolving  round  it 
in  an  orbit  presented  edgeways  to  the  Earth,  Algol 
itself  should  be  in  revolution  in  a  similar  orbit,  and 
in  the  same  plane,  and  should  therefore  during  each 
revolution  alternately  approach  and  recede  from  the 
Earth.  Such  approach  and  recession  should  pro- 
duce an  oscillation  of  the  dark  lines  in  the  spectrum 
of  the  star,  as  they  became  by  Doppler's  principle 
displaced  higher  and  lower  in  the  spectrum,  and 
precisely  such  an  oscillation  of  the  lines  as  was 
demanded  by  theory  was  actually  detected  in  twelve 
photographs  of  the  spectrum  of  Algol  taken  at 
intervals  between  1889  and  1891. 

An  oscillation  of  spectral  lines  precisely  similar 
to  that  presented  by  Algol  was  discovered  during 
the  same  period  in  Spica,  the  most  brilliant  star  in 
the  constellation  of  the  Virgin.  In  this  case  the 
complete  oscillation  was  effected  in  just  over  four 
days,  and  the  orbital  speed  of  the  star  indicated  by 
the  displacement  of  its  spectral  lines  is  56*7  miles 
per  second.  It  can,  therefore,  scarcely  be  doubted 
that,  like  Algol,  Spica  is  accompanied  by  a  dark 
companion,  but  that  the  plane  of  its  orbit  is  inclined 

1  See  pp.  22-31. 


The  Analysis  of  Starlight.  215 

to  the  line  of  sight  to  such  an  extent  that,  at  each 
conjunction,  the  dark  star  passes  either  just  over 
or  under  it,  thus  avoiding  an  eclipse.  Vogel  is 
indeed  of  opinion  that  faint  traces  of  the  spectrum 
of  the  companion  can  be  detected  in  the  photo- 
graphs. 

While  these  refined  observations  were  in  pro- 
gress at  Potsdam,  a  very  beautiful  application  of 
Doppler's  principle  was  effected  at  the  observatory 
of  Harvard  in  the  United  States.  From  1886  to 
1890  the  energy  of  Professor  E.  C.  Pickering  and 
his  assistants  was  mainly  directed  to  effecting  a 
photographic  record  of  the  spectra  of  stars  as  far  as 
the  eighth  magnitude.  The  method  adopted  con- 
sisted in  accurately  adjusting  a  photographic  plate 
at  the  principal  focus  of  the  object-glass  of  a  tele- 
scope, while  immediately  in  front  of  the  object- 
glass,  a  large  prism — known  as  an  objective  prism 
— was  fixed.  In  the  absence  of  the  prism  the 
sensibly  parallel  rays  of  a  star  would  have  been 
condensed  into  a  point-like  image  upon  the  plate, 
but  by  the  prism  the  image  was  elongated  into  a 
spectral  line.  The  method  so  far  was,  as  we  have 
seen,  that  devised  by  Fraunhofer,  who  expanded 
the  spectral  line  into  a  band  of  sensible  width  by  a 
cylindrical  lens.  In  the  work  at  Harvard,  however, 
no  cylindrical  lens  was  employed.  The  line  was 
directly  photographed,  but,  by  causing  the  tele- 
scope to  slowly  move  relatively  to  the  star  so  that 
the  spectral  line  drifted  in  a  direction  at  right  angles 
to  its  length  over  the  plate  a  band  was  produced  in 
which  the  dark  lines  were  distinctly  visible.  All 


216          Recent  Advances  in  Astronomy. 

stars  included  within  a  certain  small  area  of  the 
heavens  toward  which  the  instrument  was  directed 
formed  images,  and  therefore  spectra,  upon  the 
plate,  which  therefore  frequently  contained  a  con- 
siderable number  of  stellar  spectra.  The  method 
was  inferior  to  that  followed  at  Potsdam  in  that,  in 
the  necessary  absence  of  a  comparison  spectrum 
from  the  plate,  it  was  impossible  to  determine  the 
absolute  positions  of  lines  in  their  respective  spectra 
with  great  accuracy,  such,  for  instance,  as  would 
have  been  essential  to  the  detection  of  motion  in  the 
line  of  sight,  but  it  enabled  a  far  greater  number  of 
stars  to  be  examined  in  the  time,  the  spectra  of  over 
ten  thousand  being  in  fact  photographed  and  ex- 
amined in  four  years.  Upon  examining  several 
photographs  of  the  spectrum  of  Mizar,  the  middle 
star  of  the  three  forming  the  handle  of  the 
"  Plough"  or  the  tail  of  the  "  Great  Bear",  the 
singular  fact  appeared  that  while  upon  some  plates 
the  dark  lines  presented  a  normal  appearance,  on 
others  they  were  doubled ;  and  upon  a  more  critical 
examination  it  appeared  that  they  opened  and 
closed  with  perfect  regularity  in  successive  periods 
of  fifty-two  days.  A  simple  and  complete  explana- 
tion of  these  appearances  is  found  in  the  assumption 
that  we  are  here  presented  with  a  system  of  two 
stars,  similar  to  that  of  Algol  and  its  companion, 
except  that  in  the  case  of  Mizar  both  of  the  stars  are 
bright.  The  actual  photographed  spectrum  would 
therefore  be  a  combination  of  the  spectra  of  the  two 
stars.  The  pair  being  in  continual  revolution 
round  their  common  centre  of  mass,  at  the  instant 


The  Analysis  of  Starlight.  217 

of  their  conjunction  with  the  direction  of  the  Earth 
they  would  be  drifting  across  the  line  of  sight 
in  opposite  directions;  neither  would  be  ap- 
proaching or  receding;  the  spectral  lines  of  both, 
being  in  their  normal  positions,  would  coincide; 
and  the  appearance  of  a  single  spectrum  would 
result.  A  quarter  of  the  complete  period  of  revolu- 
tion later,  however,  one  of  the  stars  would  be  rush- 
ing towards  and  the  other  from  the  Earth,  their 
lines  would  consequently  experience  displacement 
in  opposite  directions  in  their  spectra,  and  would 
appear  as  separated.  In  another  quarter  period 
half  a  revolution  would  have  been  accomplished 
from  the  time  of  the  first  observation,  conjunction 
with  the  Earth  would  again  occur,  and  the  com- 
bined spectrum  would  once  more  assume  its  normal 
appearance.  There  can  be  little  hesitation  in 
accepting  this  explanation. 

From  the  impracticability  of  determining  the 
exact  positions  of  the  lines  in  the  spectrum  it  was 
not  possible  to  form  an  estimate  of  the  actual  speeds 
of  the  component  stars  in  the  line  of  sight,  but,  from 
the  widest  distance  to  which  the  lines  open  out  it  is 
a  simple  matter  to  determine  for  the  instant  of  their 
greatest  separation  the  speed  of  one  star  relatively 
to  the  other  in  the  line  of  sight.  It  appeared  to  be 
about  100  miles  per  second.  If  we  assume  that  the 
orbits  are  presented  edgewise  to  the  Earth,  the 
movement  of  the  stars  at  the  time  of  the  widest 
separation  of  their  spectral  lines  would  be  directly 
towards  and  away  from  the  Earth,  and  this  velocity 
would  be  the  actual  speed  of  one  star  relatively  to 


218          Recent  Advances  in  Astronomy. 

the  other.  From  the  knowledge'  of  this  relative 
velocity,  and  of  the  complete  period  of  revolution — 
in  this  case  104  days — it  is  possible  from  the  laws  of 
mechanics  and  gravitation  to  calculate  the  combined 
mass  of  the  stars.  In  the  result  a  mass  is  indicated 
of  forty  times  that  of  the  Sun.  If  the  orbits  are 
merely  inclined  to  the  direction  of  the  Earth  and  are 
not  presented  edgewise,  the  actual  relative  speeds, 
being  at  the  time  of  greatest  separation  of  the  lines 
only  in  part  directed  to  the  Earth,  must  be  greater 
than  those  assumed,  and  the  masses  of  the  stars 
must  exceed  the  value  deduced  from  the  first 
assumptions.  Since  there  are  no  means  of  deter- 
mining the  inclination  of  the  orbits  to  the  line  of 
sight,  it  becomes  therefore  only  possible  to  deter- 
mine a  limit  above  which  the  mass  of  the  double 
star  must  lie. 

From  the  spectroscopic  examination  of  an  object 
so  remote  that  its  distance  is  incapable  of  determin- 
ation, and  that  in  the  field  of  view  of  the  most 
powerful  telescope  appears  but  as  an  absolute  point 
of  light,  to  see  a  pair  of  revolving  suns ;  to  measure 
the  period  of  their  mutual  revolution ;  to  trace  over 
their  blazing  surfaces  cooler  atmospheres,  and  in 
these  to  recognize  gases  familiar  upon  the  surface 
of  the  Earth ;  and  to  assign  a  minimum  limit  to  the 
mass  of  the  entire  system,  is  an  achievement  that 
can  scarcely  fail  to  appeal  even  to  those  many  and 
most  hardened  of  sinners  against  intellectual  light 
who  would  value  scientific  investigation  only  in 
exact  proportion  to  the  monetary  equivalent  of  its 
technical  application. 


The  Red  Flames  of  the  Sun.  219 

Chapter  VI. 
The  Red  Flames  of  the  Sun. 

We  have  traced  in  a  previous  chapter  the  course 
of  discovery  resulting  from  the  spectroscopic  exam- 
ination of  the  general  light  of  the  Sun.  From 
the  time  of  Fraunhofer's  observations  to  those  of 
Angstrom,  the  light  submitted  to  examination  was 
indeed  that  of  the  Sun,  but  it  is  important  to  notice 
that  no  pains  had  been  taken  to  differentiate  the 
radiations  thrown  off  by  different  parts  of  the  Sun's 
surface.  From  the  centre,  as  well  as  from  the  edge 
of  the  disc ;  from  the  dark  spots  and  brilliant  faculas, 
as  well  as  from  the  delicate  extensions  of  its  atmos- 
pheric surroundings,  only  so  far  recognized  during 
the  brief  moments  of  totality  of  solar  eclipse ;  rays 
entered  the  spectroscope  and  mingled  their  story  in 
the  resulting  spectrum.  In  1866,  however,  Ang- 
strom for  the  first  time  adopted  a  different  method, 
one  by  which  it  became  possible  to  examine  in 
detail  the  radiations  of  different  parts  of  the  Sun's 
surface,  and  which  has  during  the  years  that  have 
followed  yielded  a  veritable  harvest  of  interesting 
and  valuable  results.  It  will  only  be  possible  in 
the  present  chapter  to  pass  under  review  very  briefly 
a  few  of  the  more  remarkable  of  these  in  their  bear- 
ing upon  the  Physics  of  the  Sun. 

Angstrom's  device  consisted  in  forming,  by  means 
of  a  convex  lens,  a  sharply-defined  image  or  picture 
of  the  Sun  upon  the  slit-plate  of  a  spectroscope. 


220          Recent  Advances  in  Astronomy. 

The  result  is  generally  and  most  conveniently 
obtained  by  adjusting  a  spectroscope  so  that  its 
collimator  lies  along  the  axis  of  an  astronomical 
telescope,  and  so  that  its  slit-plate  is  removed  from 
the  object-glass  by  a  distance  equal  to  its  focal 
length.  Under  these  conditions,  and  when  the 
telescope  is  directed  towards  the  Sun,  the  rays  from 
different  portions  of  the  Sun's  surface  are  focussed 
at  corresponding  points  upon  the  plate,  and  there 
is  formed  upon  it  a  perfectly-defined  picture  of  the 
solar  disc,  in  which  the  sun-spots  are  clearly  visible. 
The  size  of  the  image  is,  by  elementary  optical 
laws,  in  direct  proportion  to  the  focal  length  of  the 
lens  forming  it,  a  picture  of  the  Sun  an  inch  in 
diameter  necessitating  a  focal  length  of  about  9 
feet.  To  observe  the  spectrum  of  any  selected  por- 
tion of  the  Sun's  surface,  the  slit  is  so  adjusted  that 
the  image  of  that  particular  portion  falls  upon  it. 
Under  these  conditions,  the  converged  rays  from  the 
selected  region  of  the  Sun,  instead  of  illuminating 
the  plate,  pass  directly  through  the  opening  of  the 
slit,  and  travelling  through  the  spectroscope,  are 
subjected  to  analysis.  It  will  be  noticed  that  the 
method  is  essentially  the  same  as  that  applied  by 
Huggins  two  years  earlier  to  the  examination  of 
the  spectra  of  nebulae.  It  had  also  been  employed 
by  Donati  in  1864  for  the  purpose  of  examining  the 
spectrum  of  a  comet. 

It  is  important  to  observe  that  different  portions 
of  the  length  of  the  slit  being  illuminated  by  differ- 
ent parts  of  the  Sun's  surface,  the  light  filling  it 
may,  and  indeed  frequently  does,  differ  in  quality 


The  Red  Flames  of  the  Sun.  221 

in  different  parts  of  its  length.  As  in  the  resulting 
spectrum  the  light  from  each  small  part  of  the  slit 
is  spread  out  into  a  spectral  band,  the  appearance 
is  produced  of  the  several  and  frequently  differing 
spectra  of  those  portions  of  the  Sun's  surface  by 
which  the  slit  is  illuminated  arranged  as  a  series  of 
parallel  strips,  each  being  in  contact  with  those 
immediately  above  and  below  it. 

In  1868  the  new  method  was  applied  to  the  study 
of  the  "red  flames"  or  "prominences"  of  the  Sun. 
At  a  total  eclipse  of  the  Sun,  during  the  few  minutes 
at  most  in  which  the  glowing  photosphere  is  covered 
by  the  Moon,  there  are  seen,  apparently  projecting 
from  behind  the  dark  disc  of  the  Moon,  the  delicate 
appendages  of  the  Sun  known  as  the  "promin- 
ences" and  the  "corona".  The  corona  appears  as 
an  exquisitely  beautiful  and  generally  irregular  halo 
of  silvery  light  surrounding  the  black  circle  of  the 
Moon.  It  is  full  of  the  most  delicate  detail,  and 
appears  to  the  eye  to  consist  chiefly  of  streamers, 
some  of  which  frequently  extend  from  the  surface 
of  the  Sun  to  a  distance  greater  than  its  diameter. 
The  far  smaller  but  more  brilliant  "prominences" 
are  rose-tinted  projections  that  frequently  assume 
the  most  fantastic  forms,  and  occasionally  extend 
to  a  height  of  a  quarter  of  the  Sun's  diameter  from 
its  surface.  Though  at  one  time  generally  regarded 
as  belonging  to  the  Moon,  the  prominences  had 
for  some  years  been  recognized  as  of  solar  origin, 
from  the  fact  that  during  the  progress  of  an  eclipse, 
the  dark  disc  of  the  Moon  had  been  seen  to  travel 
over  them,  gradually  covering  those  in  front,  and 


222          Recent  Advances  in  Astronomy. 

at  the  same  time  unveiling  those  behind  the  direc- 
tion of  its  motion.  A  sufficient  explanation  of  their 
invisibility  upon  the  limb  of  the  uneclipsed  Sun, 
even  when  viewed  with  the  telescope,  is  that  their 
fainter  light  is  entirely  overwhelmed  by  the  far 
more  brilliant  illumination  of  the  Earth's  atmos- 
phere produced  by  the  direct  solar  rays. 

In  1868  there  occurred  an  eclipse  of  the  Sun  in 
which  the  track  of  the  Moon's  shadow  travelled 
across  India.  It  was  observed  from  several  stations 
situated  at  different  points  of  the  shadow's  path  by 
scientific  men  collected  from  all  parts  of  the  civilized 
world,  and  among  them  the  French  astronomer 
M.  Janssen,  who  had  made  special  arrangements 
to  examine  the  spectrum  of  the  prominences.  As 
they  flashed  out  at  the  instant  of  totality,  Janssen 
rapidly  brought  the  slit  of  his  spectroscope  across 
the  telescopic  image  of  one  of  the  finest,  and,  on 
applying  his  eye  to  the  instrument,  perceived  at 
once  a  number  of  bright  separated  lines,  the  certain 
indication  of  gaseous  constitution.  But  this  was 
not  all.  Janssen  was  so  impressed  with  the  extreme 
brightness  of  the  lines,  that,  as  the  prominences 
themselves  melted  from  view  in  the  reappearing 
sunlight,  he  perceived  the  possibility  of  recognizing 
them  on  the  edge  of  the  uneclipsed  Sun.  Clouds 
prevented  him  from  attempting  the  observation 
during  the  remainder  of  that  day,  but  on  the  follow- 
ing morning,  not  long  after  sunrise,  Janssen  care- 
fully searched  the  immediate  neighbourhood  of 
the  Sun's  limb  with  the  spectroscope,  and  had  no 
difficulty  in  again  recognizing  the  brilliant  lines  of 


The  Red  Flames  of  the  Sun.  223 

the  spectra  of  prominences  entirely  invisible  in  the 
telescopic  view  of  the  Sun. 

The  principle  underlying  this  discovery,  a  dis- 
covery that  initiated  an  entirely  new  application  of 
the  spectroscope,  is  extremely  beautiful,  as  well  as 
simple.  The  atmospheric  glare  in  the  direction  of 
the  Sun,  by  which  the  prominences  are  usually  con- 
cealed, is  scattered  sunlight,  and  as  such,  yields  the 
ordinary  solar  spectrum.  The  amount  of  light  dis- 
tributed over  the  whole  of  this  spectrum  is  derived 
from  that  entering  the  slit,  so  that  the  intensity  ot 
illumination,  or  brightness,  of  the  spectrum  becomes 
less  and  less  in  direct  proportion  to  its  extension  in 
length,  and  must  continue  to  do  so  until  the  dis- 
persion becomes  so  great  that  the  spectrum,  ceasing 
to  be  continuous,  breaks  up  into  a  number  of  separ- 
ate images  of  the  slit.  We  have  seen,  however, 
that  with  sunlight  this  has  never  been  effected. 
Since,  by  the  employment  of  a  number  of  prisms, 
through  all  of  which  the  light  is  transmitted  in  suc- 
cession, any  required  degree  of  spectral  extension 
may  be  obtained,  the  brightness  of  the  spectrum 
may  by  the  same  means  be  reduced  to  any  required 
extent.  With  the  prominences,  however,  this  is  not 
the  case.  From  the  fact  of  their  being  gaseous,  their 
light  consists  of  a  finite  number  of,  and  practically 
of  a  very  few,  pure  colours.  The  first  separation  of 
these  in  the  spectroscope  is,  of  course,  accompanied 
by  a  decrease  in  brightness,  but  since  the  colours 
are  now  pure,  they  are  not  further  enfeebled  to 
whatever  extent  the  spectrum  is  extended ;  the  only 
effect  of  such  extension  being  to  place  them  farther 


224          Recent  Advances  in  Astronomy. 

apart.  We  have  then  the  general  result,  that  the 
brightness  of  the  spectrum  of  the  prominences  is 
only  slightly,  and  to  a  limited  degree,  enfeebled  by 
spectral  extension;  whereas  the  spectrum  of  the 
atmospheric  glare  is  enfeebled  without  limit  in 
direct  proportion  to  spectral  extension  or  dispersion. 
After  a  certain  dispersion,  therefore,  the  general 
illumination  of  the  greatly  weakened  spectrum  of  the 
air  glare  is  no  longer  sufficiently  bright  to  conceal 
the  scarcely  reduced  light  of  the  spectrum  of  the 
prominences,  and  their  bright  lines  become  visible 
upon  the  background  of  the  enfeebled  spectrum  of 
the  sunlight  scattered  in  the  Earth's  atmosphere. 

Previously  to  Janssen's  discovery,  however,  the 
principle  by  which  it  was  effected  had  been  clearly 
recognized  by  the  English  astronomers.  It  had 
been  plainly  stated  in  1866  by  Sir  Norman  Lockyer, 
who,  at  the  time  of  the  eclipse,  had  a  powerful  spec- 
troscope in  process  of  construction  for  the  purpose 
of  attempting  its  application.  The  instrument  was 
completed  shortly  afterwards;  and,  by  its  means, 
Lockyer  detected  the  spectra  of  prominences  before 
the  news  of  Janssen's  success  had  reached  Eng- 
land. Sir  William  Huggins  had,  moreover,  already 
searched  the  limb  of  the  Sun  for  spectra  of  promi- 
nences with  a  spectroscope  of  moderate  power, 
though  without  success.  Upon  the  announcement 
of  the  discovery,  however,  he  repeated  the  obser- 
vations with  the  same  instrument;  and,  now  that 
he  was  aware  of  the  exact  part  of  the  spectrum 
toward  which  to  direct  his  attention,  had  no  diffi- 
culty in  recognizing  the  bright  lines. 


The  Red  Flames  of  the  Sun.  225 

The  bright  lines  of  the  spectra  of  prominences 
had  been  observed  by  several  astronomers  in  India 
during  the  progress  of  the  eclipse.  Although  other 
fainter  lines  had  appeared,  one  observer  having 
indeed  detected  as  many  as  nine,  by  far  the  most 
conspicuous  were  three — a  crimson,  a  green,  and  a 
yellow.  The  duration  of  the  eclipse  had  been  too 
brief  to  allow  of  an  accurate  determination  of  the 
positions  of  the  lines,  but  it  was  believed  from  their 
general  appearance  that  the  red  and  green  would 
be  found  to  be  due  to  the  radiations  of  hydrogen, 
and  the  yellow  to  those  of  sodium.  On  subse- 
quently examining  the  spectra  of  prominences  at 
leisure  in  the  uneclipsed  Sun,  the  coincidences  of 
the  red  and  green  lines  with  those  of  glowing 
hydrogen  were  established,  but  the  yellow  line  was 
found  to  be  rather  more  refrangible  than  the  yellow 
of  sodium,  and  not  to  correspond  in  position  with 
any  line  that  had  so  far  been  recognized  in  the 
spectrum  of  a  terrestrial  gas.  It  was  consequently 
assumed  to  arise  from  the  radiations  of  a  gaseous 
constituent  of  the  prominences  unfamiliar  to  terres- 
trial chemistry.  Later,  this  hypothetical  gas  re- 
ceived, at  the  suggestion  of  Frankland,  the  name 
of  helium.  For  a  long  time  subsequently,  helium 
remained  unrecognized,  save  as  a  constituent  of  the 
Sun,  stars,  and  nebulas,  until,  in  1895,  it  was  dis- 
covered by  Professor  William  Ramsay  among  the 
gases  extracted  from  a  terrestrial  mineral,  clevite. 

For  some  days  after  the  eclipse  Janssen  remained 
at  his  station  in  India,  fascinated  with  the  applica- 
tion of  the  new  discovery.  He  soon  found  that  the 

(M520)  P 


226          Recent  Advances  in  Astronomy. 

bright  lines  originally  seen  in  the  spectra  of  pro- 
minences could  be  traced,  though  to  a  far  less 
distance  from  it,  round  the  entire  limb  of  the  Sun. 
There  could  be  no  hesitation  in  ascribing  their 
continual  appearance  to  the  radiations  of  the  in- 
candescent atmosphere  of  the  Sun,  and  it  con- 
sequently appeared  that  the  prominences  were 
essentially  enormous  and  local  extensions  of  the 
solar  atmosphere.  Janssen  also  found  that  the  pro- 
minences were  essentially  unstable,  and  that  they 
were  subject  to  changes  upon  the  most  stupendous 
scale.  During  the  eclipse  an  enormous  red  flame, 
estimated  as  being  at  least  89,000  miles  in  height, 
had  been  directly  seen  upon  the  edge  of  the  Sun : 
but,  upon  the  following  day,  scarcely  a  trace  of 
bright  lines  could  be  detected  by  the  spectroscope 
in  the  place  that  it  had  occupied.  From  day  to 
day  Janssen  traced,  from  the  occurrence  and  the 
varying  distance  to  which  they  could  be  followed 
from  its  limb,  the  mighty  surging  now  recognized 
for  the  first  time  in  the  atmosphere  of  the  Sun. 

In  this,  the  earliest  method  of  studying  pro- 
minences upon  the  limb  of  the  uneclipsed  Sun, 
observation  was  confined  to  a  narrow  section  of 
the  prominence  the  image  of  which  was  at  that 
instant  formed  upon  the  slit  of  the  spectroscope. 
It  was  possible,  however,  from  the  examination 
of  a  number  of  such  sections,  to  trace  the  complete 
form  of  the  prominence.  For  this  purpose  it 
was  most  convenient  to  adjust  the  slit  so  that  it 
lay  "  radially",  or  at  right  angles  to  the  edge 
of  the  solar  image  formed  upon  the  slit  plate, 


The  Red  Flames  of  the  Sun.  227 

and  so  that  a  small  portion  of  it  penetrated  the 
image  itself.  The  appearance  in  the  spectroscope 
then  consisted  of  the  ordinary  solar  spectrum  of 
that  portion  of  its  surface  that  illuminated  part  of 
the  slit,  while  above  it  was  extended  the  bright- 
line  spectrum  of  the  solar  atmosphere  and  pro- 
minences, the  length  of  the  lines  depending  upon 
the  extension  of  the  radiating  gases  above  the  solar 
surface.  By  moving  the  spectroscope  so  that  the 
slit  travelled  round  the  solar  image  while  always 
maintaining  its  radial  position  with  reference  to  it, 
the  bright  lines  were  seen  to  contract  or  extend  as 
the  level  of  the  atmosphere  and  prominences  rose 
and  fell.  From  measurements  of  the  lengths  of  the 
lines,  it  therefore  became  possible  to  trace  the 
varying  height  of  the  incandescent  gases  produc- 
ing them,  and  thus  to  construct  the  complete  out- 
line of  a  prominence. 

In  the  early  part  of  the  following  year  a  further 
simplicity  was  effected  in  the  observation  of  pro- 
minences. In  1869  Sir  William  Huggins,  having 
by  its  spectrum  detected  a  prominence  upon  the 
limb  of  the  Sun,  carefully  adjusted  the  slit  so  as  to 
lie  within  the  image  of  the  prominence  but  just 
outside  of  that  of  the  Sun.  He  then  boldly  opened 
the  slit,  and  saw,  not  the  line  spectrum  of  the  pro- 
minence, but,  in  the  position  where  its  red  line  had 
appeared,  the  prominence  itself,  splendidly  dis- 
played as  a  cloud  of  crimson  fire. 

The  explanation  of  this  very  beautiful  discovery 
is  again  extremely  simple.  The  opened  slit  may 
be  regarded  as  a  narrow  window  directed  towards 


228          Recent  Advances  in  Astronomy. 

the  prominence.  The  light  of  the  sky  entering  the 
window  along  with  that  of  the  prominence,  is 
spread  out  into  a  very  impure  spectrum,  which, 
retaining  its  continuous  character,  is  greatly  en- 
feebled in  brightness  by  its  extension.  The  pro- 
minence light,  however,  consisting  almost  entirely 
of  three  pure  colours,  is  at  once  resolved,  but  is 
not  further  weakened.  The  crimson  rays  of  the 
prominence  are  deflected  in  block  by  the  prism, 
and,  being  scarcely  reduced  in  intensity,  convey  to 
the  eye  the  appearance  of  a  crimson  prominence 
superposed  upon  the  enfeebled  crimson  of  the  spec- 
trum of  the  atmospheric  glare.  In  a  similar  man- 
ner, a  green  picture  of  the  prominence  is  produced 
by  its  green  rays,  and  appears  upon  the  green 
region  of  the  air  spectrum,  while  each  pure  colour 
present  in  the  radiations  of  the  prominences  must, 
in  like  manner,  develop  a  picture  of  the  prominence 
in  its  own  colour.  Until  very  recently  this  con- 
stituted the  most  powerful  method  for  studying  the 
varying  forms  of  the  prominences.  From  the 
great  .visual  intensity  of  the  crimson  light,  the 
picture  formed  by  it  has  been  the  one  generally 
selected  for  observation. 

The  picture  presented  by  solar  prominences, 
when  viewed  with  fine  instrumental  means,  is  of 
such  extreme  beauty  as  to  lend  a  special  charm  to 
their  study.  As  was  first  indicated  by  Lockyer  in 
1870,  they  appear  to  be  divisible  into  two  distinct 
classes.  Those  of  the  first,  which  are  generally 
known  as  quiescent,  are  commonly  the  larger.  In 
appearance  they  closely  resemble  terrestrial  clouds 


The  Red  Flames  of  the  Sun.  229 

bathed  in  the  crimson  glory  of  sunset.  Their  soft 
outlines  experiencing  but  gradual  change,  they 
seem  to  float,  often  for  days  together,  at  a  great 
elevation  above  the  glowing  surface  of  the  Sun, 
being  sometimes  connected  with  it  by  delicate 
filaments  of  light,  but  at  other  times  entirely  sepa- 
rated from  it.  They  are  to  be  traced  round  the 
entire  limb  of  the  Sun,  and  appear  to  have  no 
direct  relation  to  sun-spots.  The  spectroscope 
shows  their  light  to  consist  almost  entirely  of  the 
radiations  of  hydrogen  and  helium. 

The  prominences  of  the  second  class,  which  are 
known  as  eruptive,  appear,  as  their  name  implies, 
to  be  the  results  of  veritable  explosions  from  be- 
neath the  cloud-surface  of  the  photosphere.  They 
are  intensely  brilliant.  They  are  subject  to  the 
most  violent  and  rapid  changes,  and  are  short- 
lived. Their  spectra  indicate,  that,  while  their  light 
is,  in  the  main,  due  to  the  glowing  hydrogen  and 
helium,  the  vapours  of  other  metals,  and  conspicu- 
ously those  of  sodium,  magnesium,  and  iron,  are 
generally  present  in  them.  They  are  clearly  con- 
nected with  the  unrecognized  physical  cause  to 
which  sun-spots  are  due,  for  they  are  most  densely 
distributed  over  the  zones  on  either  side  of  the  Sun's 
equator  to  which  spots  are  almost  entirely  limited; 
they  appear  in  greatest  abundance  at  the  approxi- 
mately regular  periods  of  eleven  years  that  mark 
the  maximum  richness  of  spot  distribution;  and,  in 
several  instances,  they  have  been  seen  in  direct  con- 
nection with  large  spots,  that  have,  by  the  slow 
rotation  of  the  Sun,  just  reached  the  edge  of  the 


230          Recent  Advances  in  Astronomy. 

disc.  When  it  has  been  possible  to  trace  the  con- 
nection most  exactly,  their  glowing  matter  has 
appeared  to  have  been  projected  from  the  edges  of 
spots,  and  later,  in  its  descent,  to  have  fallen 
towards  their  dark  and  probably  depressed  centres. 
Violent  commotions,  evidenced  by  rapid  changes 
in  appearance,  are  invariably  associated  with  pro- 
minences of  the  eruptive  class,  but  they  have 
been,  from  the  first  days  of  prominence  study,  re- 
cognized in  another  manner.  The  spectrum  of  a 
prominence  is  usually  observed  with  the  slit,  which 
is  now  very  narrow,  lying  radially  across  the  limb 
of  the  Sun's  image.  With  such  an  arrangement 
the  bright  lines  of  the  prominence  spectrum  seem, 
under  normal  conditions,  to  be  the  continuations 
of  the  corresponding  dark  Fraunhofer  lines  in  the 
solar  spectrum  appearing  immediately  below  and 
in  contact  with  them.  Frequently,  however,  this 
coincidence  is  not  maintained,  a  prominence  line 
appearing  to  be  displaced  to  one  side  or  the 
other  of  the  position  of  the  corresponding  Fraun- 
hofer line,  while  occasionally  displacements  in 
different  directions  are  observed  as  the  slit  is  ad- 
justed to  different  parts  of  the  image  of  the  same 
prominence.  The  reader  will  have  no  hesitation  in 
assigning  these  displacements  to  their  true  cause — 
to  a  Doppler  effect,  due  to  the  rush  of  glowing  pro- 
minence matter  either  directly  from  or  towards  the 
observer.  In  this  manner  velocities  in  the  line  of 
sight  have  been  recognized  of  from  200  to  300 
miles  a  second,  a  velocity  upon  much  the  same 
scale  as  that  traced  across  the  direction  of  vision 


The  Red  Flames  of  the  Sun.  231 

by  the  rapidity  of  changes  in  prominence  forms. 
From  the  detailed  method  of  observation,  however, 
spectral  changes,  due  to  movement  in  the  line  of 
sight,  are  now  more  complicated  and  fuller  of 
meaning.  Since  different  parts  of  the  slit  are 
illuminated  by  light  from  different  regions  of  a 
prominence,  and  as  each  element  of  length  of  the 
slit  gives  rise  to  a  correspondingly  narrow  strip  of 
the  spectrum,  it  frequently  happens  that,  in  different 
parts  of  its  length,  a  bright  line  displays  different 
displacements,  due  to  differing  velocities  in  the  line 
of  sight  in  the  corresponding  regions  of  the  pro- 
minence. Thus,  one  part  of  a  line  may  be  dis- 
placed to  the  red,  while  another  is  displaced  to  the 
violet,  and  the  whole  line  not  unfrequently  assumes 
a  curiously  curved  and  irregular  form.  It  is  clear 
that,  from  the  study  of  these  contorted  lines,  the 
motion  of  the  glowing  matter  in  the  line  of  sight 
may  be  determined  at  different  levels  of  the  pro- 
minence. 

For  twenty-two  years  the  method  discovered  by 
Huggins  for  observing  prominences  remained 
supreme.  In  1891,  however,  Professor  Hale  of 
Chicago  devised  an  extremely  beautiful  instrument, 
which  he  has  termed  the  spectro-heliograph,  by 
which  it  has  now  become  possible  to  record  their 
pictures  by  photography  in  a  far  more  expeditious 
and  perfect  manner.  The  principle  of  the  spectro- 
heliograph  is  as  follows.  Although,  in  the  total 
quantity  of  their  light,  the  prominences  are  so 
much  less  brilliant  than  the  glare  produced  by 
the  Sun's  rays  in  the  atmosphere  of  the  Earth, 


232          Recent  Advances  in  Astronomy. 

that  they  are,  excepting  when  the  Sun  is  totally 
eclipsed,  permanently  invisible ;  yet,  in  their 
special  radiations,  they  are  so  much  brighter, 
that,  when  observed  under  such  conditions  that 
attention  is  only  directed  to  these,  they  become 
clearly  visible.  Professor  Hale's  method  consists 
essentially  in  photographing  the  Sun  and  its 
immediate  surroundings  by  light  of  one  colour 
only,  selecting  as  that  colour  one  especially  strongly 
represented  in  the  radiations  of  prominences.  The 
prominence  ray  that  appeals  most  powerfully  to  the 
eye  is,  as  we  have  seen,  the  crimson  light  of  hydro- 
gen; but  this  is  quite  unsuitable  for  the  purpose 
of  photography,  since  it  produces  no  effect  upon  the 
ordinary  photographic  plate,  and  those  plates  that 
are  specially  prepared  so  as  to  be  affected  by  it  are 
very  slow  in  their  action.  The  more  refrangible 
green  light  of  hydrogen  might  conceivably  be 
employed ;  but,  still  better  than  either  of  these,  are 
either  of  two  deep  violet  rays  present  in  all  promin- 
ence radiations,  and  corresponding  in  their  positions 
in  the  spectrum  with  the  two  broad  dark  Fraun- 
hofer  bands  H  and  K  that  lie  almost  at  the  violet 
extremity  of  the  visible  spectrum.  Owing  to  the 
extreme  position  of  these  rays  in  the  spectrum  they 
scarcely  affect  the  eye,  and  for  this  reason  would  be 
entirely  unsuitable  for  visual  observation,  but  they 
are  extremely  energetic  in  their  action  upon  the 
photographic  plate.  Their  radiations  are,  as  we 
have  already  seen,  due  to  the  incandescent  vapour 
of  calcium.  Although  always  present  in  the  light 
of  prominences,  owing  to  the  feebleness  of  their 


The  Red  Flames  of  the  Sun.  233 

visual  effect  they  remained  undiscovered  until 
1882,  when  they  were  recognized  in  photographs  of 
the  spectra  of  prominences  seen  upon  the  edge  of 
the  dark  moon  during  the  famous  total  solar  eclipse 
of  that  year.  Although  the  dark  bands  H  and  K  are 
very  broad,  the  corresponding  violet  lines  of  the 
prominence  spectra  are  fine;  hence  there  is  this 
further  advantage  in  making  use  of  them,  that, 
since  the  colours  immediately  next  them  in  the 
spectrum  of  sunlight  are  absent  or  weakly  repre- 
sented, they  are  more  easily  isolated  for  the  pur- 
poses of  experiment.  In  practical  work  it  has  been 
found  most  convenient  to  make  use  of  the  violet 
line  that  corresponds  with  K. 

In  photographing  prominences  with  the  spec- 
tro-heliograph,  an  image  of  the  Sun  is  formed  by 
the  object-glass  of  a  telescope  upon  the  slit  plate 
of  a  spectroscope  in  the  usual  manner,  the  image 
being,  in  the  apparatus  actually  employed,  about 
2  inches  in  diameter.  The  extremely  narrow  slit, 
which  is  somewhat  longer  than  this,  lies  right 
across  the  picture,  and  is  therefore  illuminated  by 
light  from  a  narrow  strip  of  the  Sun  and  its  at- 
mospheric surroundings  on  either  side.  The 
spectrum  is  focussed  upon  a  small  screen  which 
forms  a  part  of  the  instrument,  and  it  of  course 
consists  of  a  succession  of  images  of  the  slit,  one 
formed  by  each  colour  present  in  the  light  that 
penetrates  it.  In  the  screen  there  is  a  second  slit, 
and  this  is  carefully  adjusted  so  as  to  be  in  the 
exact  position  of  the  bright  K  line.  The  violet  K 
light,  therefore,  and  that  only,  penetrates  the  second 


234          Recent  Advances  in  Astronomy. 

slit,  and  forms  an  image  of  the  first  upon  a  photo- 
graphic plate  that  is  placed  immediately  beyond. 
There  being  formed  upon  the  surface  of  the  plate 
an  exact  picture  of  the  illuminated  slit,  so  far  as  its 
K  radiation  is  concerned,  the  picture  of  the  slice  of 
the  Sun,  the  image  of  which  is  focussed  upon  the 
slit,  is  therefore  reproduced  by  these  rays.  By  the 
regular  action  of  a  water-clock,  the  slit  of  the 
spectroscope  is  now  made  to  travel  across  the  image 
of  the  Sun,  thus  successively  including  images  of 
all  portions  of  its  surface,  while,  by  a  proper 
mechanism,  at  the  same  time,  and  with  such  a 
perfectly  sympathetic  movement  that  the  K  line  of 
the  moving  spectrum  continually  coincides  in  posi- 
tion with  the  second  slit,  the  screen  in  front  of  the 
photographic  plate  is  carried  over  it.  Adjacent 
pictures  of  adjacent  strips  of  the  Sun  are  therefore 
photographed  by  K  light,  and  thus,  in  the  result,  a 
complete  picture  of  the  Sun  is  produced,  while  the 
prominences  are  so  rich  in  K  rays  that  they  appear 
beautifully  defined  upon  it.  The  passage  of  the  slit 
over  the  whole  image  of  the  Sun  occupies  but  a 
fraction  of  a  minute,  in  which  time  a  survey  is 
effected  that  would  occupy  an  observer  several 
hours  to  complete  less  perfectly  by  visual  observa- 
tion. 

Professor  Hale's  method  is  invaluable  in  recording 
not  only  prominences,  but  other  features  of  the  solar 
surface  to  which  we  have  not  as  yet  referred.  Near 
the  limb  of  the  Sun,  and  most  richly  displayed  in 
the  neighbourhood  of  sun-spots,  there  are  always 
visible  in  telescopic  observation  irregular  bright 


The  Red  Flames  of  the  Sun.  235 

masses,  commonly  forming  a  rough  network  over 
the  surface.  The  visibility  of  these  masses,  which 
have  received  the  name  of  "  faculae  ",  near  the  limb, 
combined  with  the  fact  of  their  disappearance  as  they 
are  carried  by  the  rotation  of  the  Sun  farther  on  to 
its  disc,  is  satisfactorily  explained  by  the  assumption 
that  they  are  not  appreciably  brighter  than  the  clouds 
of  the  photosphere,  but  that  they  float  at  a  higher 
level.  The  brightness  of  the  solar  disc  is  readily 
seen  through  a  telescope  to  diminish  towards  the 
limb,  a  consequence  of  absorption  exercised  upon  its 
light  by  its  atmosphere,  the  absorption  being  spe- 
cially pronounced  in  rays  coming  from  the  limb  to 
the  eye,  by  reason  of  their  oblique  passage  through 
the  atmosphere,  and  the  consequent  great  length  of 
their  path  involved  in  it.  The  faculae  being  at  a 
higher  level  than  the  general  surface,  their  light 
does  not  experience  the  effects  of  absorption  in  so 
marked  a  manner,  and  when  near  the  limb  they 
therefore  become  visible  upon  the  background  of 
the  dimmed  photosphere. 

In  1872  Professor  Young  of  New  Jersey  observed, 
that,  in  the  spectra  of  faculae,  fine  bright  lines  always 
appeared  down  the  centres  of  the  broad  bands  H 
and  K.  The  appearance  probably  indicates  that  the 
faculas  contain  the  incandescent  vapour  of  calcium 
at  a  lower  density  but  at  a  higher  temperature  than 
the  same  vapour  that,  in  the  atmosphere  at  a  lower 
level,  produces  by  its  absorption  of  the  light  of  the 
photosphere  the  bands  H  and  K.  According  to  this 
view,  the  light  of  the  photosphere  is  robbed  of  the 
H  and  K  radiations  while  traversing  the  cool  and 


236          Recent  Advances  in  Astronomy. 

dense  mass  of  calcium  vapour  lying  immediately 
above  it;  at  a  still  greater  height  more  intensely 
heated  clouds  of  the  same  vapour  in  part  restore  the 
rays,  but  the  glowing  matter  being  now  at  a  lower 
density,  the  light  is  more  truly  monochromatic,  and 
narrower  spectral  lines  are  the  result.  If,  therefore, 
it  were  possible  to  view  the  Sun  by  its  K  light,  and 
that  alone,  we  should  in  all  probability  be  able  to 
distinguish  the  faculae  not  only  near  the  limb  but 
over  the  whole  disc;  and  the  method  applied  by 
Janssen  to  the  prominences  would  probably  be  suc- 
cessful but  for  the  fact  that  these  extreme  violet 
radiations  affect  the  eye  to  so  slight  an  extent.  The 
photographs  of  the  spectro-heliograph  are,  however, 
entirely  taken  by  K  radiation,  and  it  is  not  therefore 
surprising  to  find  in  them  representations  of  faculae 
over  the  entire  picture  of  the  Sun.  It  should  be 
added,  that  when  it  is  desired  to  photograph  the 
faculas  the  operation  is  carried  out  precisely  in  the 
manner  described,  but  that  in  photographing  the 
more  delicate  prominences  upon  the  limb,  it  is  better 
to  exclude  the  light  of  the  photosphere  by  covering 
its  image  by  a  circular  disc.  In  the  resulting  pic- 
ture the  Sun  consequently  appears  black,  and  the 
whole  strikingly  resembles  a  photograph  of  the 
eclipsed  Sun. 

In  our  brief  study  of  the  work  of  the  spectroscope 
we  have  but  touched  upon  those  of  its  applications 
that  have  so  far  proved  the  most  important,  probably 
because  it  has  been  found  possible  to  interpret  them. 
It  has  been  necessary  to  pass  over  a  vast  accumula- 
tion of  its  records,  in  some  of  which  a  meaning  is 


The  Red  Flames  of  the  Sun.  237 

indicated  with  less  certainty,  but  which,  in  greater 
part,  have  utterly  baffled  rational  conjecture.  It 
cannot  be  imagined,  however,  for  a  moment  that  the 
story  of  the  spectroscope  is  as  a  tale  that  is  told. 
Year  by  year  its  record  is  accumulating,  while  year 
by  year  advance  in  other  branches  of  physical  science 
aids  in  the  task  of  dealing  effectively  with  it.  The 
story  of  its  work  during  the  past  fifty  years  is,  how- 
ever, alone  a  noble  record  of  scientific  achievement ; 
and  it  is  with  feelings  of  highest  interest  and  keenest 
expectation  that  the  astronomer  is  now  watching  its 
continual  development.  But  the  path  that  we  have 
followed  with  some  care  is  one  only  of  a  number 
along  which  knowledge  is  advancing  with  no  less 
success  and  promise  of  future  triumph.  At  the  close 
of  the  nineteenth  century  as  never  before  does  the 
music  of  Nature  resound  with  a  soul-inspiring  har- 
mony, and  never  in  the  past  have  the  paths  of  science 
appeared  so  exquisitely  attractive  to  her  children. 


Index. 


Aberration  of  light,  50,  51. 
Absorption,  mechanism  of,  177. 
Algol,  discovery  of  dark  companion, 

22,  213. 

Alpha  Centauri,  distance  of,  6. 
Alpha  Crucis,  relation  to  Milky  Way, 

90. 
Alpha  Cygni,  relation  to  Milky  Way, 

86. 
Andrews,    critical    temperature    of 

gases,  37. 

Andromeda,  nebula  in,  16. 
Angstrom,  explanation  of  dark  lines 

in  solar  spectrum,  179;  method  of 

observing  spectrum  of  Sun,  219; 

researches  on  chemistry  of   the 

Sun,  186. 
Arcturus,  motion  of,  46,  70. 

Barnard,  observations  of  Mars,  120, 

121,    129;     photographs    of   the 

Milky  Way,  66,  67. 
Becquerel,  photographs  ultra-violet 

region  of  spectrum,  167. 
Bessel,  first  detects  parallax  of  a 

star,  5,  50. 

Bifurcation  of  Milky  Way,  60. 
Boeddicker,  drawing  of  the  Milky 

Way  as  seen  by  the  naked  eye, 

89,  131- 

Break  in  the  Milky  Way,  62. 
Brewster,  discovers  telluric  lines  in 

solar  spectrum,  166. 
Bunsen   and   Kirchhoff,  researches 

in  spectrum  analysis,  180. 

Calcium,  incandescent  vapour  of,  in 


faculae,  235;  variation  of  spectrum 
under  different  physical  conditions, 
190. 

Carbon,  discovery  of  vapour  in  at- 
mosphere of  Sun,  187. 

Carbonic  acid  gas,  suggested  as  a 
constituent  of  atmosphere  of  Mars, 
142. 

Clustering  of  stars  in  neighbourhood 
of  Sun,  97. 

Coal  Sack,  in  Milky  Way,  61,  76, 
81,  83,  89. 

Collisions  between  stars,  45,  48. 

Colour,  effect  of  motion  of  source 
upon,  29. 

Colour,  relation  to  length  and  fre- 
quency of  ether  waves,  173,  175. 

Copernicus,  his  view  regarding  the 
nature  of  the  stars,  2. 

Corona,  221 ;  drawings  of,  131. 

Daguerre,  his  discoveries  in  photo- 
graphy, 167. 

Dark  matter  in  space,  possible  exis- 
tence of,  13,  32,  67. 

Dark  stars,  13,  32;  possibility  of 
detection,  22,  35. 

Diffraction  grating,  160. 

Doppler,  enunciation  of  principle 
regarding  the  effect  of  motion  of 
source  on  generated  waves,  29, 
200. 

Doppler' s  principle,  application  to 
the  discovery  of  Algol's  com- 
panion, 29. 

Douglass,  his  observations  of  Mars, 
120,  126. 


240 


Recent  Advances  in  Astronomy. 


Draper,  photographs  infra-red  region 
of  spectrum,  168;  photographs  the 
Moon,  167. 

Earth,  appearance  of,  as  seen  from 

a  planet,  114. 
Eclipse  of  1868,  222. 
Energy,  conservation  of,  45. 
Ether  of  space,  172. 

Faculae,  solar,  234,  235. 

Fizeau,  indicates  correct  application 
of  Doppler's  principle,  29,  206. 

Foucault,  observes  absorption  of  D 
light  by  gases  of  electric  arc, 
168,  170. 

Fraunhofer,  his  improvements  in  the 
spectroscope,  156;  observes  dark 
lines  in  spectra  of  stars,  157; 
observes  dark  lines  in  spectrum  of 
Sun,  156. 

Fraunhofer  lines,  first  observed  by 
Wollaston,  155;  studied  by  Fraun- 
hofer, 156. 

Gamma   Cygni,    relation   to   Milky 

Way,  87. 
Goodricke,  suggests  eclipse  theory 

of  Algol,  23. 
Groombridge  1830,  46,  99. 

Hale,  photographs  prominences  and 
faculae  in  uneclipsed  Sun,  232. 

Heliometer,  54. 

Helium,  discovered  in  clevite  by 
Ramsay,  225;  a  constituent  of 
some  nebulae,  200;  in  solar  atmo- 
sphere and  prominences,  225. 

Henderson,  detects  parallax  of  alpha 
Centauri,  6. 

Herschel,  Sir  John,  his  drawing  of 
the  Eta  Argus  nebula,  130;  his 
views  regarding  the  structure  of 
the  sidereal  system,  80;  observes 
the  relation  between  the  nebulas 
and  Milky  Way,  71. 

Herschel,     Sir    William,    observes 


antipathy  between  nebulae  and 
stars,  71 ;  observes  relation  of 
stars  to  the  Milky  Way,  68 ;  views 
regarding  the  structure  of  the 
system  of  the  stars,  73,  77 ;  views 
regarding  the  Sun,  36. 

Huggins,  Sir  William,  demonstrates 
gaseous  nature  of  certain  nebulas, 
21 ;  determines  motion  of  stars  in 
the  line  of  sight,  30,  206;  ob- 
serves variations  in  the  spectrum 
of  calcium,  191 ;  observes  spectra 
of  stars,  196;  observes  promi- 
nences in  uneclipsed  Sun,  227. 

Huygens,  enunciates  wave  theory  of 
light,  172. 

Hydrogen,  in  atmosphere  of  Sun 
and  in  solar  prominences,  225 ; 
spectrum  of,  186. 

Janssen,  observes  spectra  of  promi- 
nences in  uneclipsed  Sun,  222. 

Keeler,  his  observations  of  Mars, 

129. 
Kirchhoff,    his   researches    on    the 

chemistry  of  the  Sun,  184 ;    his 

researches  in  spectrum  analysis, 

180. 

Lane's  law  regarding  the  variation 

of  temperature  of  a  cooling  gas, 

38-40. 
Light,    possible    absorption  of,    in 

space,  79 ;  refraction  of,  147  ;  wave 

theory  of,  172-175. 
Lockyer,  Sir  Norman,  his  researches 

in  the  chemistry  of  the  Sun,  187 ; 

his  dissociation  hypothesis,  191; 

meteoritic  hypothesis,  43  (note). 
Lowell,  his  observations  on   Mars, 

166. 

Mars,  101 ;  atmosphere  of,  no,  113, 
114;  canals  of,  102,  124,  132; 
climate  of,  132,  142;  clouds  on, 
122 ;  dark  belt  surrounding  polar 


Index. 


241 


cap  on,  121 ;  distance  of,  103 ; 
gravitation  on,  103;  gray -green 
"seas"  on,  117,  120;  limb-light 
on,  no,  112;  "  a  miniature  of  the 
Earth",  102;  mass  of,  103;  op- 
positions of,  108;  orange  con- 
tinents on,  117;  phases  of,  104; 
polar  caps  of,  116;  rotation  of, 
108  ;  seasons  on,  109 ;  size  of,  103 ; 
speculations  on  possible  inhabi- 
tants, 127;  telescopic  appearance 
of,  108 ;  vapour  of  water  in  atmo- 
sphere of,  115. 

Milky  Way,  Barnard's  photographs 
of,  66,  67 ;  a  collection  of  faint 
stars,  60 ;  dark  rifts  in,  67,  81 ;  a 
definite  formation,  64,  82;  dis- 
tance of,  62,  92 ;  early  views  re- 
garding, 59;  general  appearance 
of,  59 ;  lines  of  stars  in,  67  ;  nebu- 
lous matter  in,  64,  68,  87 ;  photo- 
graphs of,  65  ;  structure  apparent 
in,  63;  telescopic  appearance  of, 
60,  64. 

Moon,  nature  of  motion  round 
Earth,  25. 

Nebulae,  constitution  of,  197;  de- 
monstration of  gaseous  nature  of, 
19,  197;  early  conjectures  as  to 
nature  of,  16;  Herschel's  views 
regarding,  16  ;  regarded  as  exter- 
nal galaxies,  18 ;  relation  of,  to 
Milky  Way,  71 ;  temperature  of, 
38. 

Newton,  Sir  Isaac,  his  analysis  of 
sunlight,  144. 

Oppositions  of  a  planet,  104. 
Orion,  nebula  in,  13;  star  streams 

in,  related  to  Milky  Way,  88. 
Oxygen,  its  apparent  absence  from 

the  atmosphere  of  the  Sun,  192. 

Parallax,  method  of  relative,  3,  52, 
56;  of  a  star,  5,  51,  52. 
(M520) 


Photographs,     of    infra    region    of 

spectrum,  168;  method  of  taking, 

of  celestial  objects,  66;  of  Milky 

Way,  66,  67;    of  spectrum,  167, 

1 68,    187;   of  ultra-violet  region 

of  spectrum,  167. 
Photography,     discovery    by     Da- 

guerre,  167;  invention  of  gelatine 

plate,  212. 

Photosphere,  solar,  36. 
Pickering,   E.  C.,  his  discovery  of 

spectroscopic  double  stars,  215. 
Pickering,  W.  H.,  his  observations 

of  Mars,  121,  126. 
Prism,  action  of,  upon  light,  149. 
Proctor,  maintains  relation  of  lucid 

stars  to  Milky  Way,  69,  86,  89 ; 

suggests     possible    structure    of 

Milky  Way,  83. 
Prominences,  solar,  228 ;  connection 

with  sun-spots,  229. 

Radiation,  mechanism  of,  176. 

Ramsay,  discovers  helium,  225. 

Ranyard,  maintains  intimate  rela- 
tion between  stars  and  Milky 
Way,  86. 

Refraction,  by  atmosphere,  51 ;  of 
light,  147. 

Resonance,  177. 

Reversal  of  spectral  lines,  first  ob- 
served by  Foucault,  168.  170; 
theory  of,  enunciated  by  Stokes, 
171-179. 

Rowland,  his  photographs  of  solar 
spectrum,  187. 

Schiaparelli,  discovers  the  canals  of 
Mars,  124. 

Selective  absorption,  135,  138. 

Simms,  applies  collimator  to  spec- 
troscope, 157. 

Sirius,  brightness  of,  8 ;  distance  of, 
7 ;  motion  of,  in  line  of  sight,  206, 
209 ;  spectrum  of,  207. 

Sky,  cause  of  appearance  of,  no. 
Q 


242          Recent  Advances  in  Astronomy. 


Spectra,  conditions  of  purity  of,  152, 
157,  158 ;  of  flames,  163,  165 ;  of 
solar  prominences,  222 ;  variation 
of,  with  different  physical  condi- 
tions, 188;  of  nebulae,  198;  of  stars, 
194 ;  of  Sun,  144. 

Spectro-heliograph,  236. 

Spectroscope,  principle  of,  153; 
prismatic,  158. 

Stars,  death  stage  of,  22;  distances 
of,  3-8,  50 ;  hypothesis  of  uniform 
distribution  of,  in  space,  77,  78, 
94,  96;  magnitudes  of,  92;  motion 
of,  45;  motion  of,  in  the  line  of 
sight,  213 ;  relation  of,  to  Milky 
Way,  68,  71,  82,  86,  91,  92,  98; 
spectra  of,  method  of  obtaining, 
194;  spectroscopic  double,  215; 
sun-like  nature  of,  2,  8. 

Stewart,  Balfour,  states  condition 
necessary  for  reversal  of  spectral 
lines,  182. 

Stokes,  Sir  Gabriel,  explains  reversal 
of  spectral  lines,  171- 179. 

Struve,  Wilhelm,  his  views  on  struc- 
ture of  sidereal  system,  81. 

Sun,  death  stage  of,  n  ;  life  history 
of,  42;  motion  of,  in  space,  47; 
source  of  heat  of,  n ;  telescopic 
appearance  of,  35. 


Sympathetic  vibration,  177. 

Telluric  lines  in  solar  spectrum,  166. 

Tyndall,  his  researches  on  selective 
absorption,  137;  illustrates  the 
theory  of  sky  formation,  no. 

Universe,  last  catastrophe  of,  49. 

Vogel,  demonstrates  existence  of 
dark  companion  of  Algol,  24,  31 ; 
measures  motions  of  stars  in  the 
line  of  sight,  212. 

Waters,  Sydney,  his  map  of  nebulae 
in  their  relation  to  Milky  Way,  72. 

Waves,  formation  of,  176 ;  of  light, 
172-175 ;  of  sound,  174,  200. 

Wollaston,  first  observes  dark  lines 
in  solar  spectrum,  156;  improves 
conditions  for  viewing  spectra, 

155- 

Wright,  of  Durham,  his  theory  re- 
garding the  structure  of  the  stellar 
system,  73. 

Young,  C.  A.,  observes  reversal  of 
calcium  lines  in  faculae,  235. 

Young,  Thomas,  establishes  the 
wave  theory  of  light,  172. 


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THE 

NATURAL  HISTORY  OF  PLANTS 

THEIR  FORMS,   GROWTH 
REPRODUCTION,   AND    DISTRIBUTION 

FROM  THE  GERMAN  OF 

ANTON  KERNER  VON  MARILAUN 

Professor  of  Botany  in  the  University  of  Vienna 
BY 

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